KR100435434B1 - A Method for Manufacturing Austenite Stainless Steel Sheet with High Strength Using Twin Roll Type Strip Caster - Google Patents

Abstract

The present invention relates to a method for producing an austenitic stainless steel sheet by cold rolling and annealing a thin plate manufactured by using a twin roll sheet casting machine for producing a thin plate directly from the molten metal. It is an object of the present invention to provide a method for producing an austenitic stainless steel sheet having high strength and excellent elongation without addition and without limitation of thickness.

The present invention relates to a method for producing an austenitic stainless steel sheet using a thin plate manufactured using a twin roll thin caster, wherein the austenitic stainless steel containing 2 to 4% of residual delta ferrite is used using a twin roll thin caster. After the stainless steel sheet is manufactured, the sheet is cold rolled by a conventional method, and then a method of manufacturing a high strength austenitic stainless steel sheet is recrystallized and annealed for 20 seconds to 60 seconds at a temperature range of 1050 ^° C to 1200 ^° C. Shall be.

Description

A method for manufacturing austenitic stainless steel sheet with high strength using twin roll type strip caster}

The present invention relates to a method for manufacturing an austenitic stainless steel sheet by cold rolling and annealing a thin plate manufactured by using a twin roll sheet casting machine for producing a thin plate directly from the molten metal. The present invention relates to a method for producing an austenitic stainless steel sheet in which the sheet is cold rolled and annealed to produce an austenitic stainless steel sheet.

Generally, austenitic stainless steel sheet manufactured by a twin roll sheet casting machine, for example, a steel plate such as 304 alloy, has a cast structure and has a columnar structure and an equiaxed crystal structure, and has a room temperature of delta ferrite of 2-4%. It contains.

Therefore, since the cast steel sheet itself has a low elongation of 40% or less, the cast steel sheet is cold rolled and recrystallized and heat treated to produce a steel sheet having a recrystallized grain structure, thereby improving mechanical properties such as tensile strength and elongation.

The mechanical properties of the basic 304 alloy have a yield strength of at least 21 kgf / mm ² , a tensile strength of 53 kgf / mm ² , and an elongation of at least 40%. In this case, the higher the yield and tensile strength possible, the better.

The higher the strength of the material, the wider the load range can be used, and even in the same load, the thickness can be reduced, thereby reducing the cost of the material used, and also has the advantage of lightening the structure. However, in the case of the 304 alloy, as the strength increases, the elongation is inversely lowered. When the elongation of the material is lowered, cracking tends to occur during processing, and cracking occurs during use, which increases the possibility of material breakage. In addition, if the elongation is small when the material is loaded, the impact is not sufficiently absorbed, and thus the use is restricted.

Therefore, the mechanical properties of 304 alloy should be designed to have high elongation while maintaining high strength as possible.

As a technique for producing a high strength 304 alloy, a method of forming nitride by adding N element to improve the strength is known.

However, in the case of the above method, there are disadvantages in that a manufacturing process for adding N is added and costs increase due to addition of another alloying element.

In addition, a method is known in which martensite is formed by cold rolling to obtain high strength by temper rolling using the properties of austenitic stainless steels in which strength is remarkably improved.

However, the above method also has a problem in that the price is increased compared to the general material due to the addition of the manufacturing process, and the flatness of the width is worsened.

Moreover, the manufacturing method of 304HB alloy which was recently announced by the Japanese metal industry as a method of improving a mechanical characteristic without adding an alloying element or changing a manufacturing process is mentioned.

In the method for producing 304HB alloy, a high strain energy is accumulated by cold rolling, and then recrystallization annealing is applied to prevent the formation and growth of microcrystal grains during recrystallization, thereby producing a steel sheet having microcrystal grains of 3 μm or less.

In general, the material is to increase the amount the more crystal grains fine strength, there is a property deterioration of the elongation is not greater, the general material grain size is 20 - 30 um because the strength of 304HB alloy is a basic specification as 41 kgf / mm ² or more under the influence of the fine grain It is more than twice and, the elongation can be obtained up to 35%.

However, in the manufacturing method of the 304HB alloy, the thickness of the manufacturable material is limited to 1.0 mm or less due to the constraint of obtaining the accumulated energy due to the high cold rolling, and the recrystallization may not be sufficiently recrystallized to 35%. As low, there is a problem that does not meet the specification.

In addition, all of the above-described conventional methods are methods for producing a steel sheet by cold rolling a hot rolled material and then performing recrystallization annealing.

MEANS TO SOLVE THE PROBLEM This invention uses the austenitic stainless steel casting plate manufactured by the thin-plate casting machine which has the characteristic of rapid solidification using the molten steel of the general component which does not add a separate element in order to improve the above-mentioned conventional problem. The research and experiment were carried out on the recrystallization annealing of the cold rolled steel sheet according to the results, and the present invention was proposed based on the results. An object of the present invention is to provide a method for manufacturing an austenitic stainless steel sheet having high strength and excellent elongation.

1 is a photograph showing the change in the microstructure according to the annealing time at 1100 ^o C annealing temperature,

(a) shows the case where the annealing time is 10 seconds,

(b) shows the case where the annealing time is 60 seconds,

(c) shows the case where the annealing time is 100 seconds,

(d) shows the case where annealing time is 300 seconds.

2 is a graph showing the change in mechanical properties with recrystallization annealing temperature and time of cold rolled steel sheet,

(a) shows the change in yield strength according to the recrystallization annealing condition,

(b) shows the change in tensile strength according to the recrystallization annealing condition,

(c) shows the change in elongation according to the recrystallization annealing condition.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention relates to a method for producing an austenitic stainless steel sheet using a thin plate manufactured using a twin roll thin caster, wherein the austenitic stainless steel containing 2 to 4% of residual delta ferrite is used using a twin roll thin caster. After the stainless steel sheet is manufactured, the sheet is cold rolled, and then recrystallized annealing for 20 seconds to 60 seconds at a temperature range of 1050 ^° C. to 1200 ^° C. to produce a high strength austenitic stainless steel sheet.

Hereinafter, the present invention will be described in detail.

The cast sheet is manufactured by a thin sheet casting machine, and recrystallized annealing heat treatment after cold rolling has two characteristics in relation to recrystallization annealing of the cold rolled material by the characteristics of the manufacturing method.

The two characteristics are the change of phase transformation behavior due to rapid solidification and the delta ferrite remaining excessively at room temperature compared with the existing hot rolled material, which is a phase having the orientation of the columnar tablet.

Since the delta ferrite can act as a nucleation position of the modified organic martensite during reverse transformation and promote recrystallization, the elongation which is increased in proportion to the recrystallization degree can be improved.

In addition, it is important that the delta ferrite particles spheroidized by cold rolling can suppress the movement of the recrystallized grain boundaries to achieve particle size refinement. Can be.

Therefore, the use of the residual delta ferrite, which is a feature of thin cast steel sheets, can achieve grain refining and rapid recrystallization rate, thereby achieving both high strength and elongation improvement simultaneously.

Therefore, the present inventors manufactured a cast sheet containing 2-4% of delta ferrite by using a twin roll sheet caster, and cold-rolled the sheet by a conventional method, and then annealing conditions, that is, annealing temperature and annealing time The experiments were carried out with various changes to study the microstructure changes and mechanical properties.

Hereinafter, the present invention will be described in more detail with reference to FIG. 1, which shows an example of experimental results.

Figure 1 shows the microstructure change with annealing time at 1100 ^o C during recrystallization annealing.

As shown in Fig. 1 (a), when the heat treatment time was very short (10 seconds), recrystallization hardly occurred. Therefore, the microstructure can be considered to be caused by rolling of the cast structure.

In the columnar structure, some of the delta ferrites remaining as dendrites are decomposed by shear deformation and remain spherical, and are stretched in the rolling direction. The equiaxed crystal structure also appears to be partially deformed, but it can be seen that the effect of the deformation due to rolling is not as large as that of the columnar tablet, and the equiaxed crystal casting structure remains as it is.

As shown in FIG. 1 (b), when the heat treatment time is further elapsed and the heat treatment is performed for 60 seconds, recrystallization in which the strained organic martensite generated during rolling is reversely transformed into austenite is started to form crystal grains. The cast structure is extinguished with the growth of recrystallized grains. Since most of the delta ferrite remained at the grain boundaries, the spheroidized delta ferrite inhibited grain growth, thus forming fine grains of about 7 μm, and also improved recrystallization rate.

In addition, as shown in Figure 1 (c), in the case of the heat treatment for 100 seconds, the cast structure is almost decomposed. The crystal grains were larger than in the case of 60 seconds, and delta ferrite, which remained relatively large, was almost decomposed and only a portion remained at the grain boundary in the form of spherical shape.

In addition, depending on the thickness direction of the thin plate, the size of the crystal grains seems to be slightly different due to the difference in the residual amount of delta ferrite, and even in the case of equiaxed crystals, the grain size is larger than other portions.

In addition, as shown in Figure 1 (d), when looking at the microstructure photograph heat treatment for 300 seconds, the longest time, it can be seen that the grain growth proceeded significantly, the cast structure is completely decomposed. It can be seen from the tissue photograph that there are almost no cast structures such as residual delta ferrite, equiaxed crystals and columnar tablets, and grown delta ferrite particles are rare in spherical form. The crystal grain size tends to be the coarser in the center where equiaxed crystals existed, and it tends to become finer as it gets closer to the surface, but the difference is very small and negligible. The grain size is about 20um and is thought to be similar to the existing conventional materials.

In this way, the present inventors conduct research and experiments in connection with the mechanical properties, such as the setting of the heat treatment conditions and the effect on the grain refinement to achieve a sufficient elongation to achieve a sufficient elongation, and based on the results, In addition, the optimum annealing heat treatment conditions can be obtained to obtain a high elongation of more than 45%.

In the present invention, when the annealing temperature is too high or the annealing time is too long, overcrystallization occurs and grains become too coarse. Furthermore, the decomposition of the delta ferrite is severe, and the effect of suppressing grain growth by the delta ferrite is weakened and the strength is increased. Will fall.

In addition, when the annealing temperature is too low or the annealing time is too short, the recrystallization does not occur sufficiently, so that martensite remains, the strength is secured, but the elongation is lowered.

Therefore, in the present invention, it is preferable to have a yield strength of 40 kgf / mm ² or more and an elongation of 45% or more by recrystallization annealing for 20 seconds to 60 seconds in the temperature range of 1050 ^° C. to 1200 ^° C.

Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example 1)

Using a twin roll sheet casting machine, a cast sheet containing 4% delta ferrite was manufactured, and the cast sheet was cold rolled at a 70% reduction rate, and then recrystallized annealed under the annealing condition of FIG. 2 to investigate mechanical properties. The results are shown in FIG.

As shown in FIG. 2, when the annealing temperature is annealed at 1050 ° C. for 60 seconds, although the yield strength is 41 kgf / mm ² or more, an elongation value of 45% or more can be obtained.

In other words, if the annealing time is shorter than 60 seconds at the annealing temperature of 1050 ℃, the recrystallization is not enough and the martensite phase remains, so the yield strength is increased, but the elongation decreases considerably, and if the heat treatment time is longer than 60 seconds, the recrystallization proceeds sufficiently. Although the elongation is increased to more than 45%, it can be seen that the yield strength decreases due to grain growth as the heat treatment time increases.

In addition, although the yield strength was 41 kgf / mm ² or more at 45 seconds at 1100 ° C., 30 seconds at 1150 ° C., and 20 seconds at 1200 ° C., the elongation value showed an excellent value of 45% or more.

As described above, this was considered to be due to an increase in elongation with increasing recrystallization rate due to residual delta ferrite, grain refining, strengthening effect of delta ferrite itself, and generation of fine inclusions by rapid solidification.

(Example 2)

The specimen without delta ferrite and the 2% and 4% specimens were heat treated at annealing temperature of 1150 ° C., and the effect of the presence of delta ferrite on the mechanical properties was investigated. The results are shown in Table 1 below.

Delta ferrite amount Mechanical property 1150 ^o C 10 sec 30 sec 60 sec 100 sec 0 % Yield strength (kgf / mm ² ) 130.5 42.5 28.6 27.2 Elongation (%) 2.8 32.6 56.4 62.1 2 % Yield strength (kgf / mm ² ) 139.2 40.5 30.5 28.5 Elongation (%) 2.4 42.3 52 64 4 % Yield strength (kgf / mm ² ) 149.9 41.2 33.6 29.7 Elongation (%) 2.7 44.8 50 65

As shown in Table 1, when there is no residual delta ferrite, it can be seen that there are few nucleation sites when recrystallization occurs, so that grains are coarse and thus mechanical properties do not meet the scope of the present invention.

As described above, the present invention has no additional addition as compared to the conventional method, and the cost of the recrystallization annealing heat treatment is shortened, as well as the cost is reduced. In terms of possible cracking and shock absorption, there is an effect of providing a very excellent austenitic stainless steel sheet.

Claims (1)

In the method for producing austenitic stainless steel sheet using a thin plate manufactured using a twin roll sheet caster, the austenitic stainless sheet containing 2-4% of residual delta ferrite is prepared using a twin roll sheet caster. After the production, cold rolled the prepared sheet, and then recrystallized annealing for 20 seconds to 60 seconds in the temperature range of 1050 ^o C-1200 ^o C of the high strength austenitic stainless steel sheet using a twin roll type cast machine Manufacturing method