KR101362388B1 - Method for producing a cold-rolled strip with a ferritic structure - Google Patents

Abstract

The present invention relates to a method for producing cold rolled strips of ferrite tissue. According to the method, molten steel forming ferrite tissue during cooling is cast into a casting strip, if necessary, the casting strip is hot rolled in-line, the hot rolled strip is wound, and cold rolled in one or more steps to form a cold rolled strip. . This type of method makes it possible to produce cold strips that minimize the risk of developing a tangerine phenomenon or leasing during the cold forming process. To achieve this, the strip is cooled between the casting process and the winding process at a cooling rate of at least 150 ° C / sec from a starting temperature of at least 1180 ° C to a maximum intermediate temperature of 1000 ° C and then at a holding temperature between 900 ° C and 1000 ° C for 10 seconds. Keep over.

Ferrite, Cold Rolled Strip, Cooling, Medium Temperature, Leasing, Tangerine

Description

METHOD FOR PRODUCING A COLD-ROLLED STRIP WITH A FERRITIC STRUCTURE}

FIELD OF THE INVENTION The present invention relates to a method for producing cold rolled strips of ferrite tissue, wherein the molten steel forming ferrite tissue is cast during cooling to produce a casting strip, and if necessary hot rolled the casting strip in-line. The present invention relates to a method for manufacturing a cold rolled strip, wherein the hot rolled strip is wound and cold rolled in one or more steps to form a cold rolled strip.

Because Ni is expensive, austenitic stainless steels are increasingly being replaced by ferritic stainless steels worldwide, and ferritic stainless steels typically contain Ni only as a production-related companion element. A method of the type described at the outset which makes this possible is known, for example, from EP 0881 305 B1. According to the prior art, it contains (up to% by weight) up to 0.12% C, up to 1% Mn, up to 1% Si, up to 0.04% P, up to 0.030% S, 16% to 18% Cr and the balance as Fe and unavoidable impurities The ferritic stainless steel is cast into a casting gap formed between the rolls of a twin roll casting machine to produce a strip. Thereafter, the strip is cooled and it is avoided to keep the strip in the austenite-ferrite transformation region during the cooling process. After cooling, the strip is wound at a temperature between 600 ° C. and martensite transformation temperature.

The winding strip is then cooled at a rate of up to 300 ° C./hour up to a temperature between 200 ° C. and ambient temperature. Finally, the winding strip is annealed in a batch type as is well known.

In the method of slab continuous casting, which is usually employed for the production of ferritic steel sheet, first, after forming the surface of the slab and reheating the slab, the slab is hot rolled with a hot strip mill to produce a hot rolled strip, and then a coil Wind up with. The hot rolled strip thus obtained is directly annealed, pickled and cold rolled in several passes. Finally, the cold rolled strip is typically polished and skin-pass rolled.

Regardless of which method is used to produce hot rolled strips, the problem with cold rolled strips made of ferritic stainless steel containing about 17% Cr is the ridging or tangerine during subsequent cold working, especially during deep-drawing. (orange peel) may occur. Leasing here means very significant linear surface defects, which, in the case of ferritic chromium steel, are aligned in the rolling direction. On the other hand, surface defects, expressed as “mandarins”, are directional and feature a rough surface appearance.

Interannealing a hot rolled strip produced by one of the known production processes between each cold rolled pass can prevent the occurrence of leasing or tangerine phenomena, although adding cost. However, this costly annealing step leads to an increase in production cost, which results in an increase in the market price of the ferritic stainless steel strip compared to the same material made of austenitic stainless steel.

It is therefore an object of the present invention to provide a cold rolled strip made of ferritic stainless steel which minimizes the risk of developing tangerine or leasing defects during the cold forming process.

This object is, in the method of the type described in the preceding paragraph, in accordance with the invention cooling the casting strip between the casting process and the winding process at least 150 ° C./sec. By cooling at a rate and holding at a holding temperature between 900 ° C. and 1000 ° C. for at least 10 seconds.

Practical onset temperatures of high-intensity cooling are typically in the range of 1180 ° C to 1270 ° C, in particular in the range of 1200 ° C to 1250 ° C. If a minimum temperature of 1180 ° C. is not achieved, a large amount of austenite can already be produced at the edges of the strip, thus making the process according to the invention unsuccessful.

Conventional steels belonging to the 10 wt% to 18 wt% Cr containing stainless steel species which form a ferrite structure and in which transformation into austenite and then again into ferrite do not occur at all during cooling starting from ferrite, the process according to the invention It is particularly suitable for carrying out this.

Such steels typically contain, in addition to Fe and unavoidable impurities, up to 0.08% C by weight (without 0%), 10% -18% Cr, up to 1% Si (without 0%), 1.5 Mn less than% (does not contain 0%), 1% or less Ni (does not contain 0%), 0.04% or less P (does not contain 0%) and 0.015% or less S (does not contain 0%) It contains. The Ni content that may be present here is not an additive amount for metallurgical purposes, but an amount incorporated into the molten steel through Ni-containing foundry ladles, converters or furnaces due to the production process. Typically the Ni content of the steel treated according to the invention ranges from 0.7% to 0.8% by weight.

The combination of strip casting, rapid cooling and the process of holding the casting strip over a sufficient time of at least 10 seconds at a temperature in the range of 950 ± 50 ° C., in particular 950 ± 20 ° C., is important for the effectiveness of the process according to the invention. Surprisingly, for cold rolled strips made of homogeneous cast strips thermally treated in accordance with the present invention, even without expensive intermediate annealing between the cold rolling stages, neither leasing nor tangerine phenomenon occurs during cold forming of the cold rolled strips. It turned out.

The steel alloys used according to the invention initially solidify with ferrite during the strip casting process. Upon cooling the solidified strip, the ferrite transforms partially into austenite between 1200 ° C and 800 ° C. This is because thermodynamically, the solubility of carbon in ferrite is very small and decreases with temperature. Carbide, which may absorb carbon, is only formed at temperatures below 900 ° C. On the other hand, the solubility of carbon in austenite is substantially greater.

During normal cooling, austenite is formed only at the grain boundaries because the carbon in the ferrite can diffuse rapidly and migrate from the inside of the grain to the edge. Thus, the ferrite grain boundary is occupied by austenite. As carbides form at temperatures below 900 ° C., the amount of austenite at grain boundaries immediately decreases. However, carbide formation proceeds relatively slowly and therefore does not occur completely, thus austenite residues remain and subsequently transform to martensite in the temperature range of 200 ° C to 300 ° C. Therefore, in the conventional processing method, residual austenite remaining at grain boundaries produces coarse cast structure.

The technical idea on which the present invention is based is to cool very rapidly to an approximate temperature (950 ° C. ± 50 ° C., especially 950 ° C. ± 20 ° C.) in which the amount of austenite is maximum. In this way, austenite formation at the grain boundary is minimized because in this case there is not enough moving diffusion distance for only carbon and substituted elements (Cr, Ni, Mn, etc.) for a short cooling time. .

At the same time, the force causing austenite formation is maximum in the holding temperature range of approximately 950 ° C., and the temperature dependent diffusion coefficient is small so that austenite particles are formed in the grains by nucleation. Due to the substantially reduced diffusion coefficient, the distribution of substitutional elements contained in each alloy does not change or the degree of change is slight (para-equilibrium). At the same time carbon supersaturation is reduced.

If the casting strip material is held at this temperature for at least 10 seconds, preferably at least 20 seconds, in the manner according to the invention, the austenite grains begin to precipitate in tissue defects in the grains. New grains that break the original cast structure develop within the ferritic matrix. As the holding time becomes longer, the particle density increases. By this precipitation mechanism, grain refinement occurs, and as a result, the cold rolled strip produced according to the present invention is insensitive to leasing and tangerine formation.

The greater the cooling rate during high-strength cooling of the cast strip according to the invention, the more austenitic formation is reliably suppressed, as intended in the present invention. Therefore, in principle, the cooling rate should be increased as much as possible. However, in the actual experiments related to this, it was confirmed that the effect used in the present invention appears reliably at a cooling rate of 150 ° C / sec ~ 250 ° C / sec.

On the basis of the basic method according to the invention described above, preferred characteristic combinations of cold rolled strips produced in each case, hot forming behavior of the cast strips obtained in each case, available plant technology or operating logistics Various modifications of the invention are possible which can be selected according to the requirements of. Thus, for example, high-strength cooling may be performed first, followed by hot rolling or initial rolling (at temperatures above 1200 ° C. in the ferrite region), followed by rapid cooling. It is also possible to pass very quickly through a two-phase range between 1200 ° C and 800 ° C. At this time, austenite is not initially formed, but the ferrite supersaturated with carbon becomes frozen. Rapid heating from a temperature below 800 ° C. to a holding temperature causes austenite to form again inside the crystal grains. In this case in particular, rapid reheating up to the holding temperature has a favorable effect on the operation results. If the heating is carried out too slowly, undesired austenite is formed at the grain boundaries, and carbon supersaturation in the grains is reduced by carbon diffusion, thereby finally fixing the original cast structure. In addition, prolonged maintenance in the temperature range of 800 ° C. to 900 should also be avoided, since actual experiments have demonstrated that carbon supersaturation decreases after approximately 100 seconds by carbide formation in this temperature range.

If the temperature reaches below 500 ° C. during cooling, for example at ambient temperature, carbides can no longer form. On the other hand, austenite grains can be formed inside the grains by freezing the supersaturated ferrite and then rapidly reheating off-line to 950 ° C.

Based on the present invention, the casting strip is cooled to an intermediate temperature of 900 ° C. to 1000 ° C. during the high strength cooling according to the present invention, thereby rapidly reaching the temperature which is important in the present invention.

According to the actual experiment, in the state where the hot rolling between the casting process and the winding process is omitted, the rapid cooling to the intermediate temperature according to the present invention, and the holding at the holding temperature between the casting process and the winding process are performed in such a method. Also, it has a favorable influence on the ridging of the obtained cold rolled strip and the insensitivity of a kumquat formation. However, in the case of homogeneous cast strips produced in a conventional manner by twin roll casting machines, it is common to at least one pass hot roll the strip between the strip casting process and the winding process, in view of the uniformity of microstructure formation and distribution characteristics. The density and precipitation rate of austenite grains are increased by hot deformation because tissue defects are introduced into the microstructure. The use of such grain refining mechanisms mainly depends on the deformation conditions and the optimal choice of cooling and heating rates. In principle, rapid heating or cooling can only be achieved for thin strips. Thus, strips produced by casting in the manner according to the invention are particularly preferred for such thermo-mechanical treatments.

When hot rolling is carried out by the method according to the invention, the strip is first cast to a thickness of 1 mm to 5 mm, in particular 2 mm to 3 mm, and then the casting strip is in-line cast of 5% to 60%, particularly 10% to 40% per pass. It is preferable to hot-roll at a reduction ratio. The action of the method according to the invention in this case allows the temperature control of the strip to be selected for the hot rolling carried out as necessary, and thus in each case depends on the ductile behavior of the treated steel or the desired combination of properties of the strip obtained. An optimally matched temperature condition is formed during hot rolling.

Thus, in the method according to the invention, a variant embodiment comprising hot rolling is possible as follows.

Modification Example 1:

-Casting casting strips;

Hot rolling the strip at a hot rolling temperature of at least 1180 ° C., typically 1180 ° C. to 1270 (initial cooling of the casting strip occurs during the hot rolling process);

Cooling at a cooling rate of at least 150 ° C./sec immediately after hot rolling to a temperature of 900 ° C. to 1000 ° C., which is equal to the intermediate and holding temperatures;

Keep the strip for at least 10 seconds at a suitable temperature between 900 ° C and 1000 ° C, in particular 950 ° C ± 20 ° C.

With regard to the first variant embodiment of the method according to the invention, only the cooling and the maintenance of the strip according to the invention are carried out after hot rolling. At the same time in this case, cooling should be initiated as soon as possible after hot rolling, within three seconds, in particular within one second, substantially after leaving the final hot-rolling stand. In this way, the casting heat of the strip can be transferred directly to the hot rolling stage, thus not only allowing high hot rolling temperatures, but also reducing the energy consumption required for temperature control of the strip to a minimum.

Modification Example 2:

-Casting casting strips;

Cooling at a cooling rate of at least 150 ° C./sec to an intermediate temperature of 900 ° C. to 1000 ° C .;

-Hot rolled strips at medium temperatures;

Hold the strip for at least 10 seconds between 900 ° C. and 1000 ° C., in particular at substantially the same temperature as the intermediate temperature or up to 950 ° C. ± 20 ° C.

In this variant of the invention, cooling is carried out to an intermediate temperature before hot rolling, and maintained at a holding temperature after hot casting of the cast strip. By hot rolling in the temperature range of 900 ° C. to 1000 ° C., additional dislocations are created in the microstructure of the hot rolled strip that serve as nucleus sites of austenite formation during holding at subsequent holding temperatures.

Modification Example 3:

-Casting casting strips;

Cooling at a cooling rate of at least 150 ° C./sec to an intermediate temperature of 900 ° C. to 1000 ° C .;

Hold the strip for at least 10 seconds between 900 ° C. and 1000 ° C., in particular at substantially the same temperature as the intermediate temperature or up to 950 ° C. ± 20 ° C .;

-Hot rolled strip at medium temperature.

According to a third variant embodiment, before the hot strip is hot rolled, it is cooled to an intermediate temperature and maintained at a holding temperature. By preserving at a holding temperature in advance, the density of austenite grains in the ferrite matrix increases, and when such a hot structure of a high density of austenite grains is hot-rolled, the dislocation density increases and a fine grain structure can be obtained during subsequent recrystallization. . Such recrystallization usually occurs by appropriate recrystallization annealing treatment, and in the production of cold rolled strips of the type under discussion, the recrystallization annealing treatment is standardized and carried out.

Modification Example 4:

-Casting casting strips;

Cooling to intermediate temperatures below 900 ° C., in particular in the range of 800 ° C., at a cooling rate of 150 ° C./sec;

-Hot rolled strips at medium temperatures;

Rapid heating of the strip to a holding temperature of 950 ° C. ± 50 ° C., in particular 950 ° C. ± 20 ° C .;

-Keep the strip at the holding temperature.

Here, hot rolling in the temperature range below 900 ° C., especially in the range of approximately 800 ° C., is performed only in the ferrite region where the yield stress is smaller than rolling in the mixed phase region. Thus, where such conditions are necessary and desirable, high deformation degrees can be achieved with reduced energy consumption and reduced roll wear.

High intensity cooling in the temperature range below 900 ° C. according to the invention offers the possibility of rolling the casting strip at a temperature substantially below 800 ° C. or carrying out further heat treatment at temperatures below 500 ° C., in particular below 400 ° C. .

Casting, hot rolling carried out if necessary, winding, and the above-described step of successively performing the cooling step up to the intermediate temperature carried out between the casting process and the winding process and the holding step at the holding temperature in succession one by one in a sequential step. The method according to the invention in several variant embodiments can be implemented particularly economically by strip casting machines.

The effects utilized by the present invention, however, also offer the possibility of discontinuously carrying out each processing step of the method according to the invention. Discontinuous implementation may be advantageous if, for example, the corresponding plant technology is available or if it is desirable to perform the processing steps at different times for logistic reasons. Accordingly, the fifth modified embodiment of the present invention is possible.

Modification Example 5:

-Casting casting strips;

Cooling at a rate of 150 ° C./second to intermediate temperatures below 900 ° C., in particular below 800 ° C .;

Cooling below 500 ° C., in particular to ambient temperature;

Rapid heating to hot-rolling end temperature;

-Hot rolled strip at hot rolled temperature;

Rapid heating of the strip to a holding temperature of 950 ° C .;

-Keep the strip at the holding temperature.

According to a fifth variant embodiment of the invention, in accordance with the present invention there is provided a casting strip cooled to an intermediate temperature of less than 900 ° C., in particular less than 800 ° C. during cooling, and it is also possible to perform cooling to a temperature as low as ambient temperature. . The cast strip is then reheated to the holding temperature. By “after” in this context it is meant that further processing steps may be carried out between the cooling and holding steps, for example hot rolling at a certain temperature, storage, splitting into sheet material, and the like.

It is also possible to cool the casting strip to ambient temperature after casting, and then, at a predetermined point thereafter, first heat to an optimum temperature and hot roll, then change to a holding temperature and hold for a necessary time.

In carrying out the invention according to the invention, if it reaches an intermediate temperature in the range of 800 ° C. to 900 ° C. upon reheating to a holding temperature, this temperature range should in principle be passed quickly for the reasons already mentioned. Thus, in one preferred embodiment of the present invention, reheating to the holding temperature takes place within 1 to 5 seconds, in particular 2 to 3 seconds, starting at each intermediate temperature.

If the intermediate temperature achieved during cooling is substantially below 800 ° C., in particular in the range of or slightly above ambient temperature, the strip should be reheated and hot rolled sufficiently rapidly for the reasons mentioned above. Thus, in the present invention, the strip must be reheated within 200 seconds, especially within 100 seconds, starting from a low intermediate temperature to a specific hot rolled temperature, typically 700 ° C. to 800 ° C. If the heating up to 800 ° C. is too slow, undesirable carbides may precipitate out. This phenomenon leads to a rapid decrease in supersaturation and thus a substantial reduction in the density of the austenite grains, as a result of which the grain refinement targeted in the present invention is not achieved.

The present invention thus offers the possibility of producing a product which is competitive in terms of both properties and price, while avoiding costly production steps. A particular advantage of the process according to the invention is that it is possible to produce cold rolled strips characterized by a homogeneous appearance and a scale-free surface. Oxides that have been attached to the casting strip can be obtained without oxides by the fact that as much of the oxide as possible is already removed during the high-strength cooling according to the invention, and therefore in the worst case still remains on the strip surface during hot rolling as required. Minimal surface defects are caused by the oxides present.

The process according to the invention therefore does not require a costly oxide descaling process which in the prior art causes periodic interruption of the continuous production process. In addition, the cost still required by the necessity of batch annealing of the cold rolled material in the prior art can be reduced by the present invention. During the implementation of the process according to the invention, cooling to temperatures below 800 ° C., for example, casting strips or hot rolled strips, for example at a temperature of 600 ° C., at a temperature that generally avoids flatness problems, for example Can be trimmed. Finally, the method according to the invention increases the hot strain in the strip casting line by temperature control, which is generally freely changeable, for example by means of a second roll stand or a smaller working roll diameter. As a result, the cold rolled strips produced according to the invention can have better deep-drawing / stretch-drawing performance compared to homogeneous cold rolled strips produced through conventional manufacturing processes. The rapid temperature change necessary to carry out the invention according to the invention can be realized in this case by strip casting techniques, since only the minimum thickness of the casting strip allows for a sufficient and rapid temperature change over the entire cross section of the strip. Because it makes.

Representative Example:

In the following representative examples (Experimental Example I to IV) carried out to demonstrate the effectiveness of the present invention, in each case, in weight percent, 0.043% C, 0.25% Si, 0.36% Mn, 0.021% P, 0.002 Ferritic steels containing% S, 16.23% Cr and 0.49% Ni were used as the base steels.

In a cast rolling facility of conventional design, in each case a casting strip was produced from a suitable molten steel, the casting strip was hot rolled to form a hot rolled strip, and finally the hot rolled strip was wound. The strip casting equipment used for this purpose included a twin roll casting machine, a hot rolled stand arranged in-line along the casting machine in the conveying direction of the casting strip, and a winding device arranged in the conveying direction of the rear of the hot rolled stand. According to certain experimental processes, high intensity water cooling apparatuses, inductively-operating ovens, and electric temperature maintenance furnaces were used.

The hot rolled strip obtained after the thermal treatment according to the invention had a fine grain structure and, unlike the micro-structure, which is conventionally referred to as a "fine", is a large number of particles in a relatively large ferrite grain (base). Particles (martensite, residual austenite, carbides) were present. Thus, the tissue as a whole is finer, but inherently more heterogeneous than conventional microcrystalline tissue. Therefore, the microstructure of the strip produced according to the present invention is characterized by a large number of particles per grain.

Conventional schemes comprising batch rolling annealing, pickling, cold rolling without intermediate annealing, gloss annealing and temper rolling, for each hot rolled strip produced according to the invention in Experimental Examples I to IV using a cast rolling facility Subsequent treatment with.

Specimens were prepared from the cold rolled strips obtained with a total strain of 70%. None of the specimens exhibited tangerine phenomenon or leasing.

Experimental Example I:

In a first variant of the method according to the invention, the thickness of the cast strip was 3 mm. After the casting strip leaving the casting gap of the twin roll casting machine reached a temperature of 1180 ° C, high-intensity water cooling was performed. The casting strip was cooled to a medium temperature of 950 ° C. in 2 seconds.

The thus cooled casting strips were kept at holding temperature for a period of 10 seconds in an induction heating oven, without interruption in a continuous production sequence, in which case the holding temperature was equal to the intermediate temperature.

According to the present invention, the thermally treated casting strip was hot rolled into a strip 2.5 mm thick.

After hot rolling, the strip was cooled to a winding temperature of approximately 550 ° C. until the winding device was reached on a run-out roller table, and the strip was wound with a coil in the winding device.

The hot rolled strip thus obtained exhibited columnar texture (approximately 100 μm wide and 500 μm long) along with the strip central region of the equiaxed crystal (grain size 150 μm). Grain boundaries were occupied by thin seams of martensite and carbides. Inside the grains there was a recrystallized region having a grain size of 20 μm. In addition, microdispersed isolated particles composed of carbide, martensite and residual austenite were present in the microstructure. The particle density was typically 15-25 particles per grain.

Experimental Example II:

In the second experimental example, a 2.8 mm thick strip was first made from the molten steel of the alloy as described above. The cast strip was maintained at a temperature of 1200 ° C. in an induction heating oven and then hot rolled into a strip 2.1 mm thick at this temperature.

Immediately after hot rolling, high-intensity water cooling was performed. In this case, the strip moving at approximately 1 m / sec was cooled to an intermediate temperature of 950 ° C. in 1 second. The strip then reached a run-out roller table having a first compartment connected to the hot rolled stand, the first compartment having a hood of 15 m or more in length such that the strip was held at a substantially constant temperature in the first compartment for 15 seconds. Ensure that it is maintained for a while. The strip was then cooled on a run-out roller table to a winding temperature of approximately 500 ° C. at which temperature the strip was finally wound up into a coil.

The microstructure of the hot rolled strip obtained in the second experimental example is, like the microstructure of the hot rolled strip obtained in the first experimental example, with the columnar structure (approximately 100 µm wide and 500 µm long), the strip central region of the equiaxed crystal. (Grain size 150 mu m) is shown. In this case, the grain boundary also showed a thin boundary tissue occupied by mattensite and carbides. Similarly, there were recrystallized regions having an average grain size of 20 mu m in the grains. In addition, similarly to the strip obtained in the first experimental example, microdispersed isolated particles composed of carbide, martensite and residual austenite were present in the microstructure. The particle density was typically 20-30 particles per grain.

Experimental Example III:

In the third experimental example, a 3 mm thick strip was first cast. After the cast strip reached a temperature of 1180 ° C., high intensity water cooling was initiated to cool the strip to an intermediate temperature of 780 ° C. in 3 seconds. The thus cooled casting strip was kept in an induction heating oven and heated to a hot rolling temperature of 800 ° C., and then hot rolled into a strip 2.5 mm thick at this hot rolling temperature. The strip was then cooled to a winding temperature of approximately 550 ° C. on a run-out roller table and wound up at this temperature.

From the strip thus obtained, a sample plate was divided at ambient temperature. It was first inductively heated to 800 ° C. and then inductively heated to 950 ° C. within 15 seconds. The heating time between 800 ° C. and 950 ° C. lasted 2 seconds.

Immediately afterwards the strip was held for 20 seconds at a holding temperature of 950 ° C. by a holding furnace. Subsequently, air cooling was performed.

The microstructure of the thermally treated hot rolled strip sample plate exhibited a strip center region (grain size 150 μm) of equiaxed crystal along with columnar texture (approximately 100 μm in width and 500 μm in length). In addition, recrystallized regions having a grain size of 20 μm were observed in the grains, and fine dispersed isolated particles composed of carbide, martensite and residual austenite existed in the microstructure. The particle density was typically 40 to 60 particles per grain.

Experimental Example IV:

Similar to Experimental Example III, a 3 mm thick strip was prepared and cooled to high strength after reaching a strip temperature of 1180 ° C. to an intermediate temperature of 780 ° C. However, unlike Experimental Example III, both the holding step and the hot rolling step at the holding temperature were performed off-line.

For this purpose, the plate was partitioned from the cast strip after cooling the casting strip to ambient temperature, which was induction heated from the ambient temperature to a hot rolling temperature of 800 ° C. within 30 seconds and hot rolled to a plate thickness of 2.4 mm. After the hot rolled sheet was repeatedly cooled, it was reheated in 3 seconds to a holding temperature of 950 ° C.

The reheated strip was held at the holding temperature for 20 seconds by a holding furnace. The strip was then air cooled.

Even in this exemplary embodiment, after holding at the holding temperature, the microstructure of the hot rolled sheet exhibits a strip center region (grain size 150 μm) of equiaxed crystal along with columnar texture (approximately 100 μm wide and 500 μm long). The grain boundary showed a thin boundary structure occupied by martensite and carbide, and a recrystallized region having a grain size of 20 μm was observed in the grain. As in the other experimental examples, there were microdispersed isolated particles consisting of carbide, martensite and residual austenite in the microstructure. The particle density was 40 to 60 particles per grain.

Claims (21)

By weight, Cr: 10.5% to 18%, C: 0.08% or less (not containing 0%), Si: 1% or less (not including 0%), Mn: 1.5% or less (including 0%) Ni: 1.00% or less (does not contain 0%), P: 0.04% or less (does not contain 0%), S: 0.015% or less (does not contain 0%) and the balance Fe and unavoidable Molten steel containing impurities and forming ferrite tissue during cooling are cast into a casting strip, hot rolled the casting strip in-line, winding the hot rolled strip and cold rolled in one or more steps to form a cold rolled strip. In the cold rolled strip manufacturing method of forming a ferrite structure, Between the casting process and the winding process, the strip is cooled at a cooling rate of at least 150 ° C / sec from a starting temperature of at least 1180 ° C to a maximum intermediate temperature of 1000 ° C and then held at a holding temperature between 900 ° C and 1000 ° C for at least 10 seconds. Cold rolled strip production method of ferrite structure. The method of claim 1, The starting temperature of the high-strength cooling is 1180 ℃ to 1270 ℃ range characterized in that the cold rolled strip production method. delete The method of claim 1, Cooling rate is 150 ℃ / sec ~ 250 ℃ / second, characterized in that the cold rolled strip manufacturing method of the ferrite structure. The method of claim 1, Method for producing a cold rolled strip of ferrite structure, characterized in that the intermediate temperature is 900 ℃ ~ 1000 ℃. The method of claim 5, Process for producing a cold rolled strip of ferrite structure, characterized in that the intermediate temperature is the same as the holding temperature. The method of claim 1, A method for producing a cold rolled strip of ferrite structure, characterized in that the cast strip is hot rolled. The method of claim 7, wherein A method for producing a cold rolled strip of ferrite structure, comprising performing the cooling step and the holding step after hot rolling. 9. The method of claim 8, A method of making a cold rolled strip of ferrite tissue, characterized in that cooling of the hot rolled strip is initiated within 3 seconds after the hot rolled strip leaves the final hot rolled stand. The method of claim 7, wherein A method of producing a cold rolled strip of ferrite structure, characterized by performing cooling to an intermediate temperature and holding at a holding temperature before hot rolling the cast strip. The method of claim 7, wherein A method for producing a cold rolled strip of ferrite structure, characterized by cooling to an intermediate temperature before hot rolling of the cast strip, and holding at a holding temperature after hot rolling. The method of claim 1, After the casting strip reaches an intermediate temperature of less than 900 ° C. during cooling, it is reheated to a holding temperature. The method of claim 7, wherein A method for producing a cold rolled strip of ferrite structure, wherein the hot roll is performed at an intermediate temperature of less than 800 ° C. The method of claim 12, A method for producing a cold rolled strip of ferrite structure, wherein reheating is performed within 1 to 5 seconds. The method according to any one of claims 1, 2 and 4 to 14, A method for producing a cold rolled strip of ferrite structure, comprising casting, hot rolling, winding, cooling to an intermediate temperature performed between a casting step and a winding step, and maintaining at a holding temperature in a discontinuous manufacturing order. 16. The method of claim 15, A method for producing a cold rolled strip of ferrite tissue, wherein the strip reaches a temperature below 500 ° C. during cooling. 17. The method of claim 16, After casting, the strip is cooled to ambient temperature. 17. The method of claim 16, A method for producing a cold rolled strip of ferrite structure, characterized in that heating to a hot rolled temperature within 200 seconds to hot roll the strip. The method according to any one of claims 1, 2 and 4 to 14, A method for producing a cold rolled strip of ferrite structure, comprising casting, hot rolling, winding, cooling to an intermediate temperature performed between a casting step and a winding step, and holding at a holding temperature in a continuous manufacturing sequence. The method according to any one of claims 1, 2 and 4 to 14, A method for producing a cold rolled strip of ferrite structure, characterized by casting the strip into a twin roll casting machine. The method according to any one of claims 1, 2 and 4 to 14, The maximum thickness of the cast strip is 6mm, characterized in that the cold rolled strip manufacturing method of the ferrite structure.