KR101650256B1 - Method for manufacturing ferritic stainless steel - Google Patents

Abstract

A ferritic stainless steel manufacturing method according to an embodiment of the present invention is a ferritic stainless steel producing method in which C is 0.02 wt% or less (excluding 0), N is 0.02 wt (Excluding 0), Si: not more than 1.0 wt% (excluding 0), Cr: 15.5 to 20.0 wt%, Nb: 0.1 to 0.55 wt%, and the balance of Fe and unavoidable impurities ; Hot rolling the slab; The hot-rolled slab is subjected to a first cold rolling and a first cold-rolling annealing to form a coarse grained grain and an unstratified grain group which is located inside the coarse grain and which is composed of one or more fine grain boundaries forming a high- ; And a second cold rolling and a second cold rolling annealing of the slab on which the unsteady grains are grown to decompose the unsteady grains into a plurality of normal grains.

Description

[0001] METHOD FOR MANUFACTURING FERRITIC STAINLESS STEEL [0002]

The present invention relates to a ferritic stainless steel and a manufacturing method thereof, and more particularly, to a ferritic stainless steel having improved formability and a manufacturing method thereof.

Generally, ferritic stainless steels produce large columnar crystals with a rotated cube orientation during continuous casting.

This columnar structure remains in the vicinity of the center layer of the rolled material without being broken in the hot rolling or hot rolling step and the hot rolled sheet of the ferritic stainless steel is formed into a coarse pancake having a grain size of 100 탆 or more with the above- Crystal grains are present.

Since these grains form a structure that deteriorates the formability of the final product, such a structure must be improved in order to improve the formability of ferritic stainless steels.

Therefore, in order to produce a ferritic stainless steel having excellent formability, it is essentially required to break down coarse grains and homogenize the microstructure and the bonded structure.

Conventionally, a ferritic stainless steel in which a plate material is laminated in two layers and subjected to hot rolling to destroy the cast structure produced during continuous casting to uniformize the microstructure and texture after hot rolling and hot-rolling, and a method of manufacturing the ferritic stainless steel Is specifically disclosed in, for example, "Ferritic Stainless Steel Excellent in Ridigiability and Method for Manufacturing the Same (Patent Document 10-2014-0017694) ".

However, since hot rolling is performed in a state in which the two plate members are superimposed, there is a problem that the two plates can be joined even when lubricating oil is sprayed between the plate members.

Further, even if the same reduction rate is applied, the thickness of the plate material varies depending on the lamination state of the plate materials, and the thickness of the plate material to be produced may be different from each other.

Korean Patent Publication No. 10-2014-0017694 (Apr.

The present invention provides a ferritic stainless steel capable of improving the moldability of the final product by modifying the microstructure of the ferritic stainless steel and a method for producing the ferritic stainless steel.

A ferritic stainless steel manufacturing method according to an embodiment of the present invention includes the steps of: C: up to 0.02 wt% (excluding 0), N: up to 0.02 wt% (excluding 0), Si: up to 1.0 wt% 15.5 to 20.0 wt% of Cr, 0.1 to 0.55 wt% of Nb, and the balance of Fe and unavoidable impurities; Hot rolling the slab; The hot-rolled slab is subjected to a first cold rolling and a first cold-rolling annealing to form a coarse grained grain and an unstratified grain group which is located inside the coarse grain and which is composed of one or more fine grain boundaries forming a high- ; And a second cold rolling and a second cold rolling annealing of the slab on which the unsteady grains are grown to decompose the unsteady grains into a plurality of normal grains.

The first cold rolling may be performed at a reduction ratio of 65 to 92%.

The first cold rolling annealing is preferably performed at a temperature of 980 to 1040 캜 for 1 hour or more.

The second cold rolling annealing may be performed at a temperature of 900 to 950 캜 for less than 1 hour (excluding 0).

The ferritic stainless steel according to one embodiment of the present invention may contain not more than 0.02 wt% of C (excluding 0), less than 0.02 wt% of N (excluding 0), less than 1.0 wt% of Si (excluding 0) 15.5 to 20.0 wt% of Nb and 0.1 to 0.55 wt% of Nb and the balance of Fe and inevitable impurities is subjected to hot rolling and then subjected to first cold rolling and first cold annealing to form coarse grains and coarse grains And a second cold rolling annealing step of performing a second cold rolling and a second cold rolling annealing on the cold-rolled and annealed steel in which the fraction of the abnormal crystal grains constituted by at least one micro-crystal grain forming a high-angle boundary with the coarse grain is 60% And the group is removed.

Preferably, the ferritic stainless steel according to an embodiment of the present invention may have an average r value of 1.6 or more, which is defined by the following formula (1).

The average r value = {r (0) + 2 r (45) + r (90)} / 4 -

In this case, in r (x °), x denotes the measurement direction relative to the rolling direction.

According to the embodiment of the present invention, there is an effect that the formability of the produced ferritic stainless steel can be improved by modifying the microstructure by decomposing the unsteady grain group into normal grains.

FIG. 1 is a view showing normal grains and abnormal grains grown in a microstructure, and FIG.
2 is a diagram for comparing the growth of the non-crystalline grain group with respect to the first cold annealing temperature of the ferritic stainless steel having the composition of the first embodiment of the present invention,
Fig. 3 is a diagram for comparing the growth of a group of abnormal grains according to the first cold annealing temperature of the ferritic stainless steel having the composition of the second embodiment of the present invention,
4 is a graph showing the growth of the abnormal crystal grain group according to the reduction rate in the first cold rolling.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It will be apparent to those skilled in the art that the present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, It is provided to let you know.

According to one embodiment of the present invention, the ferritic stainless steel contains 0.02 wt% or less of C (excluding 0), 0.02 wt% or less of N (excluding 0), 1.0 wt% or less of Si (excluding 0) To 20.0 wt% of Nb and 0.1 to 0.55 wt% of Nb, the balance of Fe and unavoidable impurities, and forming an unsteady grains group and then decomposing the unsteady grains into normal grains, thereby improving the formability.

Hereinafter, the reason for limiting the numerical value of the component content in the examples according to the present invention will be described.

The amount of carbon (C) is preferably 0.02 wt% or less (excluding 0).

The carbon (C) bonds with chromium (Cr) to form a precipitate, thereby reducing the corrosion resistance effect provided by chromium (Cr). When such precipitates are formed in the grain boundaries, chromium (Cr) It is preferable to limit the content to 0.02 wt% or less.

The amount of nitrogen (N) is limited to 0.02 wt% or less (excluding 0).

Nitrogen (N), like carbon (C), binds with chromium (Cr) to form a precipitate, which may lower the corrosion resistance of a product to be produced.

The amount of silicon (Si) is preferably 1.0 wt% or less (excluding 0).

Silicon (Si) is an element which is essential for deoxidation and is an essential element and has an effect of improving the corrosion resistance of ferritic stainless steel.

However, if it is added excessively, it is preferable to limit the content to 1.0 wt% or less because it hardens the material and deteriorates the corrosion resistance by oxygenation-bonded inclusions.

The amount of chromium (Cr) is preferably 15.5 wt% or more and 20.0 wt% or less.

Chromium (Cr) is the most important element added to ensure the corrosion resistance of ferritic stainless steels.

In the present invention, at least 15.5 wt% of chromium (Cr) is added for high corrosion resistance. However, when chromium (Cr) having an excessively high content exceeding 20.0 wt% is added, hardening of the material is caused and the production cost is increased. Therefore, it is preferable that the content is 20 wt% or less.

The amount of niobium (Nb) is preferably 0.1 wt% or more and 0.55 wt% or less.

Niobium (Nb) binds with carbon (C), nitrogen (N), iron (Fe) and chromium (Cr) to form precipitates that suppress the deterioration of corrosion resistance, It is preferable that the content is 0.1 wt% or more.

However, when niobium (Nb) is excessively added to an expensive element, the content of the niobium (Nb) is limited to 0.1 to 0.55 wt% because the raw material cost is increased.

According to an embodiment of the present invention, a ferritic stainless steel manufacturing method includes the steps of producing a slab by playing molten steel having a composition as described above to produce a ferritic stainless steel having excellent formability, by a conventional method, A step of hot rolling and a step of growing an unsteady grain group in which the hot-rolled slab is subjected to a first cold rolling and a first cold-rolling annealing, and a second cold rolling and a second cold annealing to obtain a plurality of normal grains . ≪ / RTI >

On the other hand, in the present invention, for the sake of convenience of explanation, the azimuth difference between the coarse grains and the coarse grains measured by the Electron Back-Scattered Pattern (EBSP) method is 15 degrees or more, The group of grains composed of fine grains is defined as "unsteady grains."

[Example]

Hereinafter, the present invention will be described using examples.

According to one embodiment of the present invention, a ferrite-based stainless steel manufacturing method comprises continuously casting molten steel having the above composition to produce a slab having a thickness of 150 mm, heating the slab to 1180 캜, hot rolling the steel to 5.5 mm , Cold rolling and cold rolling annealing were performed twice to modify the microstructure of the final product.

In the present invention, the modification of the microstructure is important to manifest the growth of the abnormal crystal grain group.

The growth of the abnormal crystal grain group means that a relatively coarse number of crystal grains are grown as compared with normal grain growth in which recrystallized grain grains having a uniform distribution of 100 m or less are formed at the time of cold rolling annealing.

In the present invention, as described above, the unsteady grains are defined as coarse grains containing at least one fine grains having a bearing difference of 15 degrees or more in the inside thereof, in order to more clearly express the growth of normal grains and the growth of grains of abnormal grains.

FIG. 1 is a view showing a normal grain group and an abnormal crystal grain group grown in a microstructure. In the case where the azimuth difference is 15 degrees or more, shading appears as a difference.

As shown in FIG. 1, in the normal grain growth (a), there is no crystal grain in the grain, whereas in the abnormal grain group growth (b), another grain exists in the rough grain.

C Si Cr Nb N Abnormal crystal grain group
Growing presence Example 1 0.007 0.4 19.4 0.45 0.010 ○ Example 2 0.003 0.1 17.9 0.32 0.007 ○ Example 3 0.008 0.4 18.6 0.52 0.010 ○ Example 4 0.009 1.0 18.7 0.36 0.009 ○ Example 5 0.014 0.6 15.8 0.25 0.015 ○ Example 6 0.016 0.3 19.8 0.13 0.018 ○ Comparative Example 1 0.007 0.6 15.9 0.05 0.006 × Comparative Example 2 0.008 0.5 18.3 0.07 0.005 × Comparative Example 3 0.012 0.9 19.5 0.02 0.009 ×

Table 1 is a table showing the growth of unsteady grains according to the composition ranges of various components in various examples and comparative examples when cold rolling is performed after cold rolling at a reduction ratio of 65% of the present invention.

As shown in Table 1, in the embodiment of the present invention, as described above, when the content of niobium (Nb), which is an element causing growth of the abnormal crystal grain group, satisfies 0.1 to 0.55 wt% , Whereas in the comparative examples, the content of niobium (Nb) is less than 0.1 wt%, indicating that the growth of the abnormal crystal grains does not occur.

On the other hand, the average r value widely used as a representative index indicating the formability of the material is defined by the following equation (1), and the r value is the ratio of the strain in the width direction to the strain in the thickness direction under uniaxial tensioning of the material.

The average r value = {r (0) + 2 r (45) + r (90)} / 4 -

In this case, x (r (x °) denotes the measurement direction relative to the rolling direction.

Temperature (℃) 960 980 1010 1040 1050 1080 Average r value 1.35 1.65 1.93 1.85 1.41 1.52

Table 2 shows the r value after 15% uniaxial stretching according to the first cold rolling annealing temperature of the ferritic stainless steel having the composition of Example 1 of the present invention and calculating the average r value according to the formula (1) .

As can be seen from Table 2, the first cold rolling annealing was performed at a temperature of 980 to 1040 캜 to form an abnormal crystal grain group, and then the second cold rolling and the second cold rolling annealing were performed to decompose into a plurality of normal grain grains , And the average r value is 1.6 or more, and the formability is excellent. On the other hand, when the cold-rolling annealing is performed at other temperatures, the average r value is less than 1.6, indicating that the formability is poor.

This is because the growth of the abnormal crystal grain group is not manifested at a temperature range other than 980 to 1040 占 폚.

FIG. 2 is a graph for comparing the growth of the non-crystalline grain group with respect to the first cold annealing temperature of the ferritic stainless steel having the composition of the first embodiment of the present invention. FIG. 2 is a graph for comparing the growth of an unsteady grain group according to the first cold-annealing temperature of stainless steel.

As shown in FIG. 2 and FIG. 3, when the first annealing temperature satisfies the composition range of the present invention, unsteady grain growth is exhibited in a temperature range of 980 to 1040 ° C., while amorphous grain growth is not expressed in other sections .

Therefore, it is preferable to perform annealing at a temperature of 980 to 1040 占 폚 in the first cold rolling annealing according to an embodiment of the present invention.

Niobium (Nb) is combined with carbon (C), nitrogen (N), iron (Fe) and chromium (Cr) during the ferritic stainless steel manufacturing process to form precipitates. And has an effect of obstructing growth of normal grains.

As described above, a small number of crystal grains can be grown in a state where the growth of the normal crystal grains is obstructed, so that the growth of the abnormal crystal grains can be manifested.

Accordingly, in the present invention, it is preferable that the precipitate is formed by optimizing the content of niobium (Nb) so that the growth of the abnormal crystal grain group can be manifested to cause the abnormal crystal grain growth to occur.

The effect of such a precipitate can be predicted by measuring the content of niobium (Nb) contained in the precipitate during the first cold rolling and annealing.

In the present invention, the total amount of niobium (Nb) contained in the precipitate produced according to the first cold annealing temperature of the ferritic stainless steel having the composition of Example 1 of the present invention was measured and shown in Table 3, The total amount of niobium (Nb) contained in the precipitates of the ferritic stainless steels having the composition shown in Table 4 was measured.

Temperature (℃) 960 980 1010 1040 1050 1080 Total amount of Nb extracted as precipitate 0.09 0.31 0.32 0.28 0.01 0.00

Temperature (℃) 960 980 1030 1080 Total amount of Nb extracted as precipitate 0.02 0.13 0.18 0.00

As shown in Tables 2 and 3, in the ferritic stainless steels having the composition of Example 1 of the present invention, the total amount of niobium (Nb) in the precipitates in the temperature range of 980 to 1040 ° C 0.25 wt% or more, whereas when the temperature exceeds 1040 ° C, the total amount of niobium (Nb) in precipitates decreases sharply.

This is because, when annealing is performed in excess of 1040 占 폚, the precipitate is reused as a base material.

On the other hand, even if cold rolling annealing is performed at a temperature of 980 to 1040 캜 in general, normal grain growth occurs without disturbance of the precipitates when the cold rolling annealing time is short (about 0 to 5 minutes). In the present invention, It is obvious that the first cold rolling annealing time is given for a long time, that is, a heat treatment time of one hour, and a phenomenon occurs at a time longer than the first cold annealing time.

Further, the present invention realizes the growth of the abnormal crystal grain group by controlling the reduction ratio (%) defined by the following formula (2) during the first cold rolling.

(%) = {(Initial thickness - final thickness) / initial thickness} x 100 (2)

Reduction rate (%) 45 55 65 75 78 82 85 88 92 95 Average r value 1.25 1.58 1.93 1.88 1.85 1.87 1.82 1.75 1.62 1.48

Table 5 shows the average r value according to the reduction rate in the first cold rolling.

As shown in Table 5, when the reduction ratio (%) in the first cold rolling is 65 to 92%, the average r value is 1.6 or more, indicating that the formability is improved.

4 is a graph showing the growth of the abnormal crystal grain group according to the reduction rate in the first cold rolling.

As shown in FIG. 4, by controlling the reduction ratio (%) during the first cold rolling to 65 to 92%, the fraction of the abnormal crystal grain group becomes higher and the formability can be improved. On the other hand, The fraction of the unstratified crystal grains sharply decreases and the formability is poor even if the second cold rolling and the second cold annealing are performed.

On the other hand, in the present invention, the unsteady grain group formed by the first cold rolling and the first cold annealing is subjected to the second cold rolling and the second cold annealing to decompose into a plurality of normal grains, thereby improving the formability of the material.

At this time, it is preferable that the second cold rolling annealing is controlled in a temperature range of less than 900 to 950 占 폚, and the annealing time is less than 1 hour. This is because the group of abnormal crystal grains formed by the first cold rolling and the first cold annealing can be decomposed into a plurality of crystal grains to improve the formability. When the second cold rolling annealing is performed at a temperature of 950 캜 or more for one hour or more, And the moldability may be deteriorated due to growth of the group again.

Thus, a ferrite stainless steel having an average r value of 1.6 or more and excellent moldability can be produced.

Although the present invention has been described with reference to the accompanying drawings and the preferred embodiments described above, the present invention is not limited thereto but is limited by the following claims. Accordingly, those skilled in the art will appreciate that various modifications and changes may be made thereto without departing from the spirit of the following claims.

Claims (6)

C: not more than 0.02 wt% (excluding 0), N: not more than 0.02 wt% (excluding 0), Si: not more than 1.0 wt% (excluding 0), Cr: 15.5 to 20.0 wt%, Nb: 0.1 to 0.55 wt% And the remaining Fe and inevitable impurities;
Hot rolling the slab;
The hot-rolled slab is subjected to a first cold rolling and a first cold-rolling annealing to form a coarse grained grain and an unstratified grain group which is located inside the coarse grain and which is composed of one or more fine grain boundaries forming a high- ; And
And a second cold rolling and a second cold rolling annealing the slab on which the unsteady grains are grown to decompose the unsteady grains into a plurality of normal grains.
The method according to claim 1,
Wherein the first cold rolling is performed at a reduction ratio of 65 to 92%.
The method according to claim 1,
Wherein the first cold rolling annealing is performed at a temperature of 980 to 1040 캜 for 1 hour or more.
The method according to claim 1,
Wherein the second cold rolling annealing is performed at a temperature of 900 to 950 占 폚 for less than 1 hour (excluding 0).
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