KR102277730B1 - Stainless steel including boron with excellent hot ductility and tensile property and method of manufacturing the same - Google Patents

Abstract

With respect to boron-containing stainless steel with excellent hot workability and tensile properties, which can secure neutron shielding performance through boron addition and improve hot workability and tensile properties by suppressing the formation of hypereutectic boride precipitates by adding cobalt, and a method for manufacturing the same start
The boron-containing stainless steel having excellent hot workability and tensile properties according to the present invention has carbon (C): 0.15 wt% or less, manganese (Mn): 2.0 wt% or less, silicon (Si): 1.0 wt% or less, chromium (Cr) : 18 to 20% by weight, nickel (Ni): 12 to 15% by weight, boron (B): 0.20 to 2.75% by weight, cobalt (Co): 3 to 10% by weight, and remaining iron (Fe) and unavoidable impurities characterized in that

Description

Boron-containing stainless steel with excellent hot workability and tensile properties and a manufacturing method therefor {STAINLESS STEEL INCLUDING BORON WITH EXCELLENT HOT DUCTILITY AND TENSILE PROPERTY AND METHOD OF MANUFACTURING THE SAME}

The present invention relates to boron-containing stainless steel and a method for manufacturing the same, and more particularly, to suppress the formation of hyper-eutectic boride precipitates by adding cobalt while ensuring neutron shielding performance through boron addition. The present invention relates to a boron-containing stainless steel excellent in hot workability and tensile properties with improved workability and tensile properties, and a method for manufacturing the same.

Since boron-containing stainless steel has excellent neutron shielding performance and corrosion resistance, it is used in nuclear fuel storage containers for storing nuclear fuel used in nuclear power plants.

Such boron-containing stainless steel is a eutectic alloy of austenite and boride [(Cr, Fe) ₂ B] from a financial point of view, and in addition to brittleness (brittle) boride itself, boride and austenite There was a problem in that the hot workability was poor because the difference in strength at the interface of the phases was large and cracks easily propagated.

As a related prior document, there is Korean Patent Application Laid-Open No. 10-2013-0074218 (published on Jul. 4, 2013), which describes an austenitic stainless steel having excellent corrosion resistance and a method for manufacturing the same.

An object of the present invention is to ensure neutron shielding performance through boron addition, while suppressing the formation of hypereutectic boride precipitates by adding cobalt to improve hot workability and tensile properties. To provide a manufacturing method thereof.

The boron-containing stainless steel having excellent hot workability and tensile properties according to an embodiment of the present invention for achieving the above object is carbon (C): 0.15 wt% or less, manganese (Mn): 2.0 wt% or less, silicon (Si): 1.0 wt% or less, chromium (Cr): 18 to 20 wt%, nickel (Ni): 12 to 15 wt%, boron (B): 0.20 to 2.75 wt%, cobalt (Co): 3 to 10 wt% and the rest It is characterized in that it contains iron (Fe) and unavoidable impurities.

The stainless steel may further contain one or more of sulfur (S): 0.04 wt% or less, phosphorus (P): 0.06 wt% or less, and nitrogen (N): 0.01 wt% or less.

It is more preferable that the cobalt (Co) is added in an amount of 3.5 to 4.5 wt%.

In addition, the stainless steel has a tensile strength (TS) of 720 MPa or more and an elongation (EL) of 4% or more.

The stainless steel has a fraction of hypereutectic boride precipitates of 1 vol% or less of the total volume of the stainless steel.

[Here, the hypereutectic boride precipitate is (Fe, Cr) ₂ B precipitate.]

A method for manufacturing boron-containing stainless steel having excellent hot workability and tensile properties according to an embodiment of the present invention for achieving the above object is (a) carbon (C): 0.15 wt% or less, manganese (Mn): 2.0 wt% or less, Silicon (Si): 1.0 wt% or less, Chromium (Cr): 18 to 20 wt%, Nickel (Ni): 12 to 15 wt%, Boron (B): 0.20 to 2.75 wt%, Cobalt (Co): 3 to Casting molten steel containing 10% by weight and the remaining iron (Fe) and unavoidable impurities; (b) hot and cold rolling the cast steel; and (c) annealing and pickling the cold rolled steel.

In step (a), the steel may further contain one or more of sulfur (S): 0.04 wt% or less, phosphorus (P): 0.06 wt% or less, and nitrogen (N): 0.01 wt% or less .

After step (a), the fraction of hypereutectic boride precipitates has 1 vol% or less of the total volume of the stainless steel.

[Here, the hypereutectic boride precipitate is (Fe, Cr) ₂ B precipitate.]

In addition, after step (c), the stainless steel has a tensile strength (TS) of 720 MPa or more and an elongation (EL) of 4% or more.

According to the present invention, boron-containing stainless steel having excellent hot workability and tensile properties and a method for manufacturing the same can secure neutron shielding performance through boron addition, and suppress the formation of hypereutectic boride precipitates by adding cobalt, resulting in hot workability and tensile properties can improve

As a result, in the boron-containing stainless steel having excellent hot workability and tensile properties according to the present invention and the method for manufacturing the same, _{the fraction of hypereutectic (Fe, Cr) 2} B precipitates is produced at 1 vol% or less of the total volume, thereby improving hot workability. It is possible to secure high strength and high toughness at the same time by securing a tensile strength (TS) of 720 MPa or more and an elongation (EL) of 4% or more.

1 is a SEM photograph showing a specimen according to Example 2 and Comparative Example 4.
2 is an SEM photograph showing the specimens according to Example 3 and Comparative Example 5;
3 is a SEM photograph showing the specimens according to Example 4 and Comparative Example 6.
4 is a phase balance diagram for specimens according to Comparative Examples 3 and 2;
5 is a graph showing the phase distribution according to the change in the addition amount of boron and cobalt in the 17Cr-12Ni component system.
6 is a graph showing the phase distribution according to the change in the addition amount of boron and cobalt in the 20Cr-12Ni component system.

Advantages and features of the present invention, and methods for achieving them, will become apparent with reference to the embodiments described below in detail in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but will be embodied in various different forms, only this embodiment allows the disclosure of the present invention to be complete, and common knowledge in the technical field to which the present invention belongs It is provided to fully inform the possessor of the scope of the invention, and the present invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout.

Hereinafter, with reference to the accompanying drawings, a boron-containing stainless steel excellent in hot workability and tensile properties according to a preferred embodiment of the present invention and a manufacturing method thereof will be described in detail as follows.

hot workability and boron-containing stainless steel with excellent tensile properties

The boron-containing stainless steel having excellent hot workability and tensile properties according to an embodiment of the present invention aims to have a tensile strength (TS) of 720 MPa or more and an elongation (EL) of 4% or more.

To this end, the boron-containing stainless steel having excellent hot workability and tensile properties according to an embodiment of the present invention has carbon (C): 0.15 wt% or less, manganese (Mn): 2.0 wt% or less, silicon (Si): 1.0 wt% Hereinafter, chromium (Cr): 18 to 20% by weight, nickel (Ni): 12 to 15% by weight, boron (B): 0.20 to 2.75% by weight, cobalt (Co): 3 to 10% by weight and the remaining iron (Fe) ) and unavoidable impurities.

In addition, the stainless steel may further contain one or more of sulfur (S): 0.04 wt% or less, phosphorus (P): 0.06 wt% or less, and nitrogen (N): 0.01 wt% or less.

At this time, it is more preferable that the cobalt (Co) is added in an amount of 3.5 to 4.5 wt%.

In addition, the stainless steel preferably has a fraction of hypereutectic boride precipitates of 1 vol% or less of the total volume of the stainless steel.

[Here, the hypereutectic boride precipitate is (Fe, Cr) ₂ B precipitate.]

Hereinafter, the role and content of each component included in the boron-containing stainless steel having excellent hot workability and tensile properties according to an embodiment of the present invention will be described as follows.

carbon (C)

Carbon (C) is an effective element for increasing strength by solid solution strengthening.

The carbon (C) is preferably added in a content ratio of 0.15 wt% or less, more preferably 0.05 to 0.15 wt%, based on the total weight of the stainless steel according to the present invention. When the amount of carbon (C) added is less than 0.05 wt%, it is difficult to properly exhibit the effect of increasing the strength. Conversely, when the amount of carbon (C) added exceeds 0.15 wt%, it is easily combined with a carbide-forming element such as chromium (Cr) effective for corrosion resistance, thereby lowering the chromium (Cr) content around the grain boundaries, thereby reducing corrosion resistance.

Manganese (Mn)

Manganese (Mn) is a deoxidizing element that lowers the oxygen concentration in molten steel and is a necessary component for refining.

The manganese (Mn) is preferably added in a content ratio of 2.0 wt% or less, more preferably 0.5 to 2.0 wt%, based on the total weight of the stainless steel according to the present invention. However, when the addition amount of manganese (Mn) exceeds 2.0% by weight, the corrosion resistance is lowered, the residual of the induced radioactivity increases.

silicon( Si )

Like manganese (Mn), silicon (Si) is a deoxidizing element added to lower the oxygen concentration in molten steel. In addition, when silicon (Si) is formed of an oxide, it serves to improve corrosion resistance.

The silicon (Si) is preferably added in a content ratio of 1.0 wt% or less, more preferably 0.1 to 1.0 wt%, based on the total weight of the stainless steel according to the present invention. However, when a large amount of silicon (Si) is added in excess of 1.0 wt %, there is a problem in that weldability and hot workability are deteriorated.

chrome( Cr )

Chromium (Cr) is an essential component of stainless steel, and is an effective element for the formation of a passivation film required to secure the surface corrosion resistance of stainless steel.

The chromium (Cr) is preferably added in a content ratio of 18 to 20% by weight of the total weight of the stainless steel according to the present invention. When the amount of chromium (Cr) added is less than 18% by weight, it may be difficult to secure heat resistance. Conversely, when the amount of chromium (Cr) is excessively added exceeding 20% by weight, since it is a ferrite phase generating element, an excessive δ-ferrite phase remains, thereby deteriorating hot workability.

nickel( Ni )

Nickel (Ni) is an essential component of stainless steel along with chromium, and is an element that stabilizes the austenite phase.

The nickel (Ni) is preferably added in a content ratio of 12 to 15% by weight of the total weight of the stainless steel according to the present invention. When the addition amount of nickel (Ni) is less than 12 wt%, it is difficult to properly exhibit the above effect. Conversely, when nickel (Ni) is added in excess of 15% by weight, the effect is saturated, so the cost is high, and the liquidus temperature of the steel is lowered, thereby preventing shrinkage pore defects during casting. cause it to occur.

Boron (B)

Boron (B) is an essential element in order to improve the neutron shielding performance.

The boron (B) is preferably added in a content ratio of 0.20 to 2.75% by weight of the total weight of the stainless steel according to the present invention. When the amount of boron (B) added is less than 0.20% by weight, the amount of boron (B) added is insufficient, so that the neutron shielding performance cannot be properly expressed. Conversely, when the amount of boron (B) added exceeds 2.75% by weight, it may _{cause cracks during casting due to excessive generation of boride {(Cr, Fe) 2} B} precipitates, strength, wear resistance, and hot workability. There is a problem of lowering the back.

Cobalt (Co)

Cobalt (Co) serves to improve hot workability and tensile properties by suppressing the formation of boride precipitates during the casting process.

The cobalt (Co) is preferably added in a content ratio of 3 to 10% by weight, more preferably 3.5 to 4.5% by weight, based on the total weight of the stainless steel according to the present invention. When the amount of cobalt (Co) added is less than 3% by weight, it may be difficult to properly exhibit the effect of improving hot workability and tensile properties because the amount is insignificant. Conversely, when the amount of cobalt (Co) added exceeds 10% by weight, the increase in the amount of cobalt, which is relatively expensive compared to nickel, boron, etc., may act as a factor to increase the manufacturing cost of stainless steel without further increasing the effect. can't

Sulfur (S)

Since sulfur (S) is a component that reduces hot workability, it is preferable to add as little as possible. Therefore, the sulfur (S) is limited to 0.04% by weight or less of the total weight of the stainless steel according to the present invention.

Phosphorus (P)

Phosphorus (P) is an unavoidable impurity contained in steel, which causes intergranular corrosion during pickling or inhibits hot workability. Therefore, phosphorus (P) is limited to 0.06% by weight or less of the total weight of the stainless steel according to the present invention.

Nitrogen (N)

Nitrogen (N) is an austenite stabilizing element, and is an element that simultaneously improves high-temperature strength and corrosion resistance. However, when the amount of nitrogen (N) is excessively added in excess of 0.01 wt%, the hot workability is deteriorated. Therefore, the nitrogen (N) is limited to 0.01 wt% or less of the total weight of the stainless steel according to the present invention.

hot workability and method for producing boron-containing stainless steel with excellent tensile properties

The method for manufacturing boron-containing stainless steel having excellent hot workability and tensile properties according to an embodiment of the present invention is (a) carbon (C): 0.15 wt% or less, manganese (Mn): 2.0 wt% or less, silicon (Si): 1.0 Weight % or less, chromium (Cr): 18 to 20 wt%, nickel (Ni): 12 to 15 wt%, boron (B): 0.20 to 2.75 wt%, cobalt (Co): 3 to 10 wt% and the remainder of iron Casting molten steel containing (Fe) and unavoidable impurities; (b) hot and cold rolling the cast steel; and (c) annealing and pickling the cold rolled steel.

In step (a), the steel may further contain one or more of sulfur (S): 0.04 wt% or less, phosphorus (P): 0.06 wt% or less, and nitrogen (N): 0.01 wt% or less .

In the step (c), in the annealing process after cold rolling, the annealing temperature greatly affects residual stress relief, crystal grain refinement, and fine carbonitride precipitation.

In this step, the annealing temperature is preferably carried out under the conditions of 900 ~ 1,100 ℃. When the annealing temperature is less than 900° C., coarse carbides may be generated and the structure may become non-uniform. Conversely, when the annealing temperature exceeds 1,100° C., the grains may be extremely coarse.

The boron-containing stainless steel having excellent hot workability and tensile properties according to the embodiment of the present invention produced by the above process can secure neutron shielding performance through boron addition, and suppress the formation of hypereutectic boride precipitates by adding cobalt Thus, hot workability and tensile properties can be improved.

As a result, in the boron-containing stainless steel with excellent hot workability and tensile properties produced by the method according to the embodiment of the present invention, _{the fraction of hypereutectic (Fe, Cr) 2} B precipitates is less than 1 vol% of the total volume, so that hot workability is achieved. high strength and high toughness can be simultaneously secured by securing a tensile strength (TS) of 720 MPa or more and an elongation (EL) of 4% or more.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail through preferred embodiments of the present invention. However, this is presented as a preferred example of the present invention and cannot be construed as limiting the present invention in any sense.

Content not described here will be omitted because it can be technically inferred sufficiently by those skilled in the art.

1. Specimen Preparation

After preparing an ingot having the chemical composition shown in Tables 1 and 2 in a vacuum induction melting furnace, hot and cold rolling of the ingot was performed, and then annealed at a temperature of 1,000 ° C. Specimens according to 16 and Comparative Examples 1 to 8 were prepared.

[Table 1] (Unit: wt%)

[Table 2] (Unit: wt%)

2. Evaluation of mechanical properties

Table 3 shows the results of evaluation of mechanical properties of the specimens according to Examples 1 to 4 and Comparative Examples 1 to 6. In addition, FIG. 1 is an SEM photograph showing the specimens according to Example 2 and Comparative Example 4, FIG. 2 is an SEM photograph showing the specimens according to Examples 3 and 5, and FIG. 3 is Example 4 and Comparative Example 6 It is an SEM photograph showing the specimen according to

[Table 3]

As shown in Tables 1 and 3, it can be seen that the specimens according to Examples 1 to 4 satisfy both the tensile strength (TS) of 720 MPa or more and the elongation (EL) of 4% or more corresponding to the target values.

On the other hand, in Comparative Examples 1 to 3 and Comparative Examples 5 to 6, the elongation rate satisfies the target value, but the tensile strength did not reach the target value.

In addition, the specimen according to Comparative Example 4 was added with a similar content of boron to the specimen according to Example 2, but due to the non-addition of cobalt, the tensile strength was significantly lower than that of Example 2 and the elongation was sharply decreased. It can be seen that .

1 to 3, the specimens according to Examples 2 to 4 _{suppressed the generation of hypereutectic (Fe, Cr) 2} B precipitates due to the addition of cobalt, so that compared to the specimens according to Comparative Examples 4 to 6 It can be seen that eutectic (Fe, Cr) ₂ B precipitates are significantly reduced.

At this time, it can be seen that, in the specimens according to Comparative Examples 4 to 6, cobalt was not added and a large amount of hypereutectic _{(Fe, Cr) 2 B precipitates of coarse size were precipitated.}

Meanwhile, Table 4 shows the results of the hypereutectic boride precipitate fraction for the specimens according to Examples 5 to 16 and Comparative Examples 7 to 8. At this time, the hypereutectic boride precipitation fraction was calculated and shown by thermodynamic calculation under the condition of 1257°C.

[Table 4]

As shown in Tables 2 and 4, in the specimens according to Examples 5 to 16, _{it can be confirmed that the hypereutectic (Fe, Cr) 2} B precipitate fraction satisfies 1 vol% or less.

On the other hand, in the specimen according to Comparative Example 7, _{the inhibitory effect of hypereutectic (Fe, Cr) 2} B precipitates was not properly exhibited due to the small amount of cobalt added at 1 wt%, and the hypereutectic (Fe, Cr) ₂ B precipitate fraction was decreased. It was confirmed that it exceeds 1 vol%.

In addition, in the specimen according to Comparative Example 8, despite the large amount of cobalt added as 10 wt% due to the excessive amount of boron added at 2.80 wt%, the hypereutectic (Fe, Cr) ₂ B precipitate fraction exceeded the target value. you can check what you did.

Meanwhile, FIG. 4 shows the phase balance diagrams for the specimens according to Comparative Examples 3 and 2.

As shown in FIG. 4 , in the case of the specimen according to Example 2 compared to the specimen according to Comparative Example 3, more cobalt was added to the boron-containing stainless steel so that the eutectic point moved to the right during cooling. It is possible to improve the elongation by increasing the fraction of austenite (proeutectic austenite).

5 is a graph showing the phase distribution according to the change in the addition amount of boron and cobalt in the 17Cr-12Ni component system, and FIG. 6 is a graph showing the phase distribution according to the change in the addition amount of boron and cobalt in the 20Cr-12Ni component system.

5 and 6, the phase distribution according to the amount of boron and cobalt added at 1257° C. is shown. As the amount of cobalt increases, the liquid area increases, and vice versa, the liquid + superinformation boride area. (Liquid + M ₂ B) decreases.

For example, if boron is added at 2.5wt% and cobalt is added at 10wt% and 2wt% by drawing a line, the case where cobalt is added at 2wt% is more eutectic than when cobalt is added at 10wt%. (M ₂ B) content is increased.

In other words, as the amount of cobalt added increases, the liquid + superfine boride region (Liquid + M ₂ B) decreases, and as a result, as the amount of cobalt increases, the fraction of hypereutectic boride decreases, thereby improving elongation.

In the above, the embodiments of the present invention have been mainly described, but various changes or modifications can be made at the level of those skilled in the art to which the present invention pertains. Such changes and modifications can be said to belong to the present invention as long as they do not depart from the scope of the technical spirit provided by the present invention. Accordingly, the scope of the present invention should be judged by the claims described below.

Claims (9)

Carbon (C): 0.03 to 0.15 wt%, Manganese (Mn): 0.5 to 2.0 wt%, Silicon (Si): 0.1 to 1.0 wt%, Chromium (Cr): 18 to 20 wt%, Nickel (Ni): 12 ~ 15% by weight, boron (B): 0.20 to 2.75% by weight, cobalt (Co): 3 to 10% by weight and the remaining iron (Fe) and unavoidable impurities, boron-containing stainless steel with excellent hot workability and tensile properties.
According to claim 1,
The stainless steel is
Sulfur (S): 0.04 wt% or less, phosphorus (P): 0.06 wt% or less, and nitrogen (N): 0.01 wt% or less Contained stainless steel.
According to claim 1,
The cobalt (Co) is
Boron-containing stainless steel having excellent hot workability and tensile properties, characterized in that it is added in an amount of 3.5 to 4.5 wt%.
According to claim 1,
The stainless steel is
A boron-containing stainless steel having excellent hot workability and tensile properties, characterized in that it has a tensile strength (TS) of 720 MPa or more and an elongation (EL) of 4% or more.
According to claim 1,
The stainless steel is
A boron-containing stainless steel having excellent hot workability and tensile properties, characterized in that the fraction of hypereutectic boride precipitates is 1 vol% or less of the total volume of the stainless steel.
[Here, the boride precipitate is (Fe, Cr) ₂ B precipitate.]
(a) Carbon (C): 0.03 to 0.15 wt%, Manganese (Mn): 0.5 to 2.0 wt%, Silicon (Si): 0.1 to 1.0 wt%, Chromium (Cr): 18 to 20 wt%, Nickel (Ni) ): 12 to 15% by weight, boron (B): 0.20 to 2.75% by weight, cobalt (Co): casting molten steel containing 3 to 10% by weight and the remaining iron (Fe) and unavoidable impurities;
(b) hot and cold rolling the cast steel; and
(c) annealing and pickling the cold rolled steel;
A method for producing boron-containing stainless steel having excellent hot workability and tensile properties, comprising:
7. The method of claim 6,
In step (a),
In the river
Sulfur (S): 0.04 wt% or less, phosphorus (P): 0.06 wt% or less, and nitrogen (N): 0.01 wt% or less A method for making a stainless steel containing stainless steel.
7. The method of claim 6,
After step (a),
A method for producing boron-containing stainless steel having excellent hot workability and tensile properties, characterized in that the fraction of hypereutectic boride precipitates is 1 vol% or less of the total volume of the stainless steel.
[Here, the boride precipitate is (Fe, Cr) ₂ B precipitate.]
7. The method of claim 6,
After step (c),
The stainless steel is
A method for producing boron-containing stainless steel having excellent hot workability and tensile properties, characterized in that it has a tensile strength (TS) of 720 MPa or more and an elongation (EL) of 4% or more.

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