KR101523229B1 - Metal material with improved low temperature property and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a metal material with an improved low temperature property, and a manufacturing method thereof. The metal material can be used in low temperatures as the metal material induces a ductile fracture due to an increase in fracture toughness at low temperatures. According to an embodiment of the present invention, the metal material with an improved low temperature property has at least one notch recessed on the surface and one or more micro-cracks crossing the notch in a width direction processed on a lower end of the notch.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a metal material having improved low-

The present invention relates to a metal material and a manufacturing method thereof, and more particularly, to a metal material having improved low-temperature characteristics which can be safely used at low temperatures by inducing soft fracture by increasing fracture toughness at low temperatures, .

Generally, in the case of facilities used in areas such as cold or polar regions, parts applied to ships, vehicles, or facilities or parts used in low temperature conditions, a material having high low-temperature fracture toughness is required.

At low temperature, tensile strength and yield strength increase, and elongation and cross - sectional shrinkage become larger at room temperature and smaller at lower temperature. This means that the ductility decreases as the temperature decreases.

Low temperature brittleness is a phenomenon in which the tensile strength and hardness are high and the impact value is low at a low temperature of 0 ° C or less. When a metal material having a body center cubic (BCC) crystal structure or a dense hexagonal (HCP) This means that, despite the failure, ductile to brittle transition occurs with a decrease in temperature, resulting in cleavage fracture.

Cleavage fracture is defined as rapid fracture along a particular crystallographic plane. The plane on which cleavage failure occurs is the plane with the largest interplanar spacing, the plane with small interatomic bonding force is the progressive surface of the crack, and the metal with the body center cubic (BCC) crystal structure is the cleavage fracture plane. The cleavage path of the cleavage fracture proceeds from the polycrystalline material into the mouth, and when the boundary is encountered, the crack propagation path changes at a constant angle depending on the orientation relationship of the two neighboring crystals, and the crack propagation path is broken. The direction of cracking along the cleavage plane is perpendicular to the maximum principal stress when viewed macroscopically. The cleavage fracture occurs when the plastic deformation is limited, and the metal of the face center cubic (FCC) crystal structure does not cause cleavage failure because the slip system suitable for the soft fracture operates at low temperature. However, the body center cubic (BCC) (HCP) crystal structure is limited in the slip system at low temperatures, so that cleavage failure occurs.

On the other hand, the ductile-brittle transition phenomenon in which the impact absorption energy is lowered as the temperature decreases is caused by the brittle fracture progressing into the mouth when plastic deformation is difficult to occur due to the competition between the plastic deformation and cleavage crack generation and growth. The body-centered cubic (BCC) crystal structure metal has a limited slip system, resulting in a pronounced ductile-brittle transition phenomenon.

At present, metal materials used in a low-temperature environment are mainly nickel alloys or stainless steels excellent in low-temperature characteristics, and these materials are disadvantageous in that they are expensive.

In order to prevent the occurrence of safety accidents due to brittle fracture at a low temperature, a low-temperature fracture of a metal material, which is a body-centered cubic (BCC) crystal structure, Researches for improving the toughness have been continued. The material characteristics are improved through fine grain refinement mainly by changing the alloying element and composition ratio, or internal factors of the material such as heat treatment.

However, improving the low-temperature characteristics of the material through the development of alloying elements and heat treatment methods has limitations in improving the strength and fracture toughness at the same time due to the internal characteristics of the material itself, There is a problem that applicability in various fields is low compared with an increase in manufacturing cost due to the addition of the resin.

KR 10-2010-0117958 A (November 11, 2010 open) KR 10-2012-0011292 A (2012.02.07 open) KR 10-1996-7004080 A (August 31, 1996 open)

SUMMARY OF THE INVENTION The present invention has been conceived to solve the problems as described above, and one embodiment of the present invention provides a metal material having improved strength and low temperature fracture toughness by external processing while maintaining the characteristics of the material, and a manufacturing method thereof .

According to a preferred embodiment of the present invention, at least one notch is formed on the surface of the notch, and at least one fine crack is formed on the lower end of the notch so as to cross the notch in the width direction. A metal material with improved properties is provided.

At this time, the notch is formed on at least one surface of the raw material which is processed by cold rolling or cold forging.

At this time, it is preferable that the notch has a wedge-shaped or arcuate cross-sectional shape.

Further, the notch includes a pair of inclined surfaces and a boundary portion in which the pair of inclined surfaces are in contact with each other.

At this time, the fine cracks are processed in a direction perpendicular to the longitudinal direction of the boundary portion.

In addition, a plurality of fine cracks may be formed spaced apart from each other.

At this time, the raw material is preferably a metal having a body-centered cubic (BCC) crystal structure.

A notch forming step of forming a notch on the surface; And a crack machining step of machining at least one fine crack at a lower end of the notch so as to cross the width direction of the notch.

Here, the notch is preferably formed in a wedge-shaped or arcuate cross-sectional shape.

At this time, before the notch forming step, it may further include a push-down processing step of pushing down the raw material, and the raw material may be processed by cold rolling or cold forging.

The notch includes a pair of inclined surfaces and a boundary portion where the pair of inclined surfaces contact with each other, and the fine cracks are processed in a direction perpendicular to the longitudinal direction of the boundary portion.

At this time, the notch can be formed by press working, cutting work, electric discharge machining or laser machining.

The fine cracks may be formed by cutting, discharging, or laser processing.

At this time, it is preferable that a plurality of fine cracks are spaced apart from each other.

The raw material is preferably made of a metal having a body-centered cubic (BCC) crystal structure.

According to the metal material having improved low-temperature characteristics and the manufacturing method thereof according to the embodiment of the present invention, the characteristics inside the metal material are maintained and the strength and low-temperature fracture toughness are improved by external machining, .

Also, it can be applied to various industrial fields such as LNG tank, icebreaker, and plant used in a low temperature environment, and it is possible to prevent safety accident by improving safety at low temperature.

Particularly, by improving the low-temperature characteristics of a low-cost body-centered cubic (BCC) crystal structure metal, a metal material having a high tensile strength and low-temperature fracture toughness can be produced at low cost, and is also applicable to a large structure made of a thick metal material.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a temperature-stress graph for illustrating a ductile-brittle transition according to an embodiment of the present invention. FIG.
2 is a schematic view of a metal material having improved low temperature characteristics according to an embodiment of the present invention.
3 is a graph comparing the tensile strengths along the rolling direction and the longitudinal direction of the notch.
4 is a graph comparing absorbed energy along the rolling direction and the longitudinal direction of the notch.
5 is a flowchart of a method of manufacturing a metal material with improved low temperature characteristics according to an embodiment of the present invention.
6 (a) is a perspective view of a conventional Charpy impact test specimen as a comparative example.
6 (b) is a wavefront photograph of the specimen shown in Fig. 6 (a). Fig.
7 (a) is a perspective view of a specimen for a Charpy impact test in which a fine crack is machined according to an embodiment of the present invention as a test example.
7 (b) is a wavefront photograph of the specimen shown in Fig. 7 (a).
8 is a graph showing the Charpy impact test results of Comparative Examples and Test Examples.
9 is a wavefront photograph of a comparative example and a test example according to a temperature change.
10 is a schematic view showing a notch of an arc-shaped sectional shape according to another embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, preferred embodiments of a metal material having improved low-temperature characteristics and a manufacturing method thereof according to the present invention will be described with reference to the accompanying drawings. In this process, the thicknesses of the lines and the sizes of the components shown in the drawings may be exaggerated for clarity and convenience of explanation.

In addition, the terms described below are defined in consideration of the functions of the present invention, which may vary depending on the intention or custom of the user, the operator. Therefore, definitions of these terms should be made based on the contents throughout this specification.

In addition, the following embodiments are not intended to limit the scope of the present invention, but merely as exemplifications of the constituent elements set forth in the claims of the present invention, and are included in technical ideas throughout the specification of the present invention, Embodiments that include components replaceable as equivalents in the elements may be included within the scope of the present invention.

Example

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph of temperature versus stress for describing soft-brittle transition according to an embodiment of the present invention.

Generally, when a metal material is subjected to a pressing process such as cold rolling or cold forging, the yield strength curve shifts to the right (σ _Y1 → σ _Y2 ) in the temperature-stress graph shown in FIG. 1 and the brittle strength the brittle strength line moves downward (σ _B1 → σ _B2 ) and increases as the ductile-brittle transition temperature (DBTT) moves from point (1) to point (2).

However, according to the metal material having improved low-temperature characteristics according to the embodiment of the present invention, as the yield strength curve shown in Fig. 1 moves to the right (? _Y1 ? _Y2 ), the brittle- σ _B1 → σ _B3 ) and the ductile-brittle transition temperature (DBTT) moves from the point (1) to the point (3) which is lower than the point (1), the strength of the metal material and the low temperature fracture toughness can be simultaneously increased .

2 is a schematic view of a metal material having improved low-temperature characteristics according to an embodiment of the present invention.

The metallic raw material having improved low-temperature characteristics according to an embodiment of the present invention is not particularly limited in its kind. However, in the case of a material having a body-centered cubic (BCC) crystal structure or a dense hexagonal (HCP) crystal structure that exhibits a brittle fracture tendency at low temperatures, compared with a material having a face-centered cubic (FCC) crystal structure, Can be further improved.

Hereinafter, the present invention will be described as an example of the present invention in which a low-alloy steel having a body-centered cubic (BCC) crystal structure and a low value is selected as a raw material.

According to an embodiment of the present invention, a notch 10 is formed on the surface of a raw material so as to improve the low-temperature fracture toughness of the metal material, and a fine crack 20 Is processed. At this time, it is preferable that the raw material is in a state in which the strength is improved by the pressing process.

As shown in FIG. 2, at least one notch 10 is formed on the surface of a raw material composed of a low alloy steel. At this time, it is preferable that the raw material is processed by cold rolling or cold forging so that the strength is improved, and the notch 10 is formed on the adjacent surface of the pressed surface.

For example, when the raw material is cold-rolled as shown in Fig. 2, the notch 10 is formed on one or both of the adjacent side surfaces 40 on both sides of the rolling surface 30 in the rolling direction .

The notch 10 may be formed by various methods such as, for example, a pressing process, a cutting process, a discharge process, or a laser process. It is preferable that the notch 10 is appropriately selected in consideration of the specifications of the material and the notch and the manufacturing efficiency.

The notch 10 may be formed in a wedge shape having a V-shaped cross-section including a pair of inclined surfaces 11 opposed to each other and a boundary portion 12 in which these inclined surfaces 11 are in contact with each other at the lower end have.

Here, the specifications such as the number of the notches 10, the interval between the notches 10, the width, the length and the depth of the notch 10 are appropriately selected in accordance with the necessity in consideration of the specifications of the raw materials, the internal characteristics such as strength and toughness .

At this time, it is preferable that the longitudinal direction of the boundary portion 12 of the notch 10 is formed in a direction perpendicular to the rolling direction.

Figs. 3 and 4 show the results obtained when the 9Cr-2W steel (FM steels), which is a martensitic structure, is cold-rolled at a reduction ratio of 60%, the rolling direction and the longitudinal direction of the notch In the graph of FIG. 3 and 4, when the rolling direction is perpendicular to the longitudinal direction of the notch (indicated by the solid line), the tensile strength and the absorbed energy Of the total population.

Referring again to FIG. 2, a fine crack 20 is machined at the lower end of the notch 10 to cross the notch 10 in the width direction. That is, the fine cracks 20 are processed from the lower end of one side slope 11 of the notch 10 to the lower side of the other side slope 11 through the boundary 12. At this time, it is preferable that a plurality of fine cracks 20 are formed parallel to each other in the longitudinal direction of the boundary portion 12 by cutting, discharging or laser processing. The fine cracks 20 may be inclined at a predetermined angle with respect to the longitudinal direction of the boundary portion 12 of the notch 10 and are preferably elongated in a direction perpendicular to the longitudinal direction of the boundary portion 12 .

It goes without saying that the specifications such as the number of the fine cracks 20, the interval between the fine cracks 20, and the width and length of the fine cracks 20 can be appropriately selected according to need.

As described above, the metal material having been subjected to the cold rolling process, at least one or more notches 10 being formed on the surface thereof, and at least one or more fine cracks 20 formed on the lower end of the notch 10, The low-temperature fracture toughness is improved.

Here, the strength can be improved by cold rolling or cold forging of the raw material. In particular, in the case of increasing the strength by increasing the potential of the metal structure by cold rolling, by adjusting the reduction ratio and the rolling direction The mechanical properties of the raw material can be easily controlled.

The improvement of the low-temperature fracture toughness is achieved by the notch 10 and the fine crack 20. In general, the crack acts as a defect at a low temperature to cause fracture. However, by controlling the number and direction of cracks, The toughness can be increased.

That is, according to the embodiment of the present invention, the stress concentrated on the cracks and defects in the raw material is dispersed by the notches 10 artificially formed on the surface, and the stress concentrated on the boundary portion 12 of the notch 10 Is further dispersed in the fine cracks 20 processed in the direction perpendicular to the boundary portion 12, and the toughness of the metal material is rapidly increased. At this time, it is possible to control the toughness improving effect by controlling the number and direction of the fine cracks 20.

5 is a flowchart illustrating a method of manufacturing a metal material having improved low-temperature characteristics according to an embodiment of the present invention. Hereinafter, a method of manufacturing a metal material having improved low-temperature characteristics according to an embodiment of the present invention will be described in order with reference to FIG.

Squeeze  Processing step ( S10 ):

The metallic raw material which is to improve the low temperature characteristic is subjected to rolling processing to improve the strength. At this time, it is preferable that a low alloy steel having a body-centered cubic (BCC) crystal structure and a low value is selected as a raw material, but other metallic materials may be selected.

The pressing process can be performed by a cold rolling or a cold forging process. Preferably, the strength of the raw material is increased by increasing the potential of the raw material by a cold rolling process. For example, when the low alloy steel is cold-rolled at a reduction ratio of about 60%, the strength improvement effect of 40% or more can be obtained. That is, by controlling the reduction rate and the rolling direction, the mechanical properties such as the strength of the raw material can be appropriately controlled.

Notch  Molding step ( S20 ):

At least one notch (10) is formed on the surface of the raw material subjected to the pressing processing. In the case of the cold rolled raw material, it is preferable to form the notch 10 on the adjacent face 40 perpendicular to the rolling direction with respect to the rolled face 30.

At this time, the notch 10 is recessed on the surface of one side adjacent surface 40 of the rolling surface 30 or on the surface of the both side adjacent surface 40 in the form of a wedge having a V- And includes a pair of inclined surfaces 11 and an edge portion 12 in the form of an edge in which the lower ends of the inclined surfaces 11 are in contact with each other. The longitudinal direction of the boundary portion 12 is preferably a direction perpendicular to the rolling direction.

The notch 10 is preferably formed by press working. However, the notch 10 may be manually formed by using a punch or the like when necessary, or the notch 10 may be formed by cutting or electric discharge machining or laser processing. The standard such as the number of the notches 10 and the interval between the notches 10, the width, the length and the depth of the notch 10 can be appropriately selected as required.

crack  Processing step ( S30 ):

A fine crack 20 is formed at the lower end of the notch 10 and extends across the notch 10 to the both inclined surfaces 11. At this time, it is preferable that the fine cracks 20 are processed into a groove shape by electrical discharge machining. If necessary, the fine cracks 20 can be cut by a separate cutting tool or formed by laser machining.

It is preferable that the fine cracks 20 extend from the lower end of one side slope 11 to the lower end of the other side slope 11 through the boundary 12 and are machined away from each other in the longitudinal direction of the boundary 12, 12 in the vertical direction.

Here, the specifications such as the number and direction of the fine cracks 20, the spacing between the fine cracks 20, and the width and length of the fine cracks 20 are appropriately adjusted according to need, taking into account the effect of improving the toughness due to the fine cracks 20 Can be selected.

As described above, when a plurality of fine cracks 20 are machined across the boundary 12 of the notch 10, the stress concentrated on the boundary 12 is dispersed through the fine cracks 20, Resulting in an improved effect. In other words, ductile fracture occurs at low temperature and thus safe use is possible.

FIG. 6A is a perspective view of a conventional Charpy impact test specimen as a comparative example, and FIG. 7A is a perspective view of a Charpy impact test specimen in which a fine crack is processed according to an embodiment of the present invention as a test example.

Hereinafter, with reference to FIG. 6 (a) and FIG. 7 (a), test examples of the metal material with improved low-temperature characteristics according to an embodiment of the present invention and its manufacturing method will be described.

First, as shown in FIG. 6 (a), a test piece for a Charpy impact test in which a notch 10 of 'V' shape is cut at an intermediate portion is prepared as a comparative example. Cracks 20 in the direction perpendicular to the longitudinal direction of the boundary portion 12 are formed in the boundary portion 12 where the inclined surfaces 11 of the notch 10 are in contact with each other, ) Was prepared as a test sample.

At this time, the specimen had a tempered martensite structure and was made of a low alloying steel material which was quenched at 870 ° C. and then tempered at 630 ° C. for 8 hours to austenizing heat treated. The chemical composition of low alloy steel materials is shown in Table 1 below.

ingredient C Si Mn P S Ni Cr Mo Cu Nb V Al B wt% 0.086 0.26 0.86 0.004 0.003 3.78 0.23 0.005 0.009 0.014 0.026 0.016 0.0002

Then, each specimen was cooled to a low temperature and Charpy impact test was performed. The specimen was cut at the boundary (12).

FIG. 6 (b) is a photograph of the wavefront cut at -140 ° C. of the specimen shown in FIG. 6 (a), and FIG. 7 to be.

At this time, brittle fracture occurs due to stress concentration at the boundary portion 12 of the notch 10, as shown in the dotted line in Fig. 6 (b). On the other hand, it can be seen that the specimen prepared as the test specimen has ductile fracture due to the improvement of the fracture toughness due to the stress dispersion at the boundary portion (12).

That is, in the case of the comparative example, brittle fracture occurs at a low temperature due to the concentration of stress in the region indicated by the ellipse in the transverse direction in Fig. 6 (a). On the other hand, in the case of the test example, the discharge machined fine crack 20 interferes with the stress concentration, and the stress is dispersed in the longitudinal oval area shown by the one-dot chain line around the fine crack 20 in Fig. 7 (a) , A large amount of energy is absorbed by the energy dispersion at the time of fracture, so that fracture toughness is increased and ductile fracture occurs.

Therefore, even in the case of the static failure mode, the conventional material as in the comparative example at low temperature causes sudden brittle fracture. However, as in the experiment example according to the embodiment of the present invention, the micro crack 20 is formed at the lower end of the notch 10 The material formed is susceptible to ductile fracture at low temperatures and cracks progress slowly, resulting in high safety.

Fig. 8 is a graph showing the Charpy impact test results of the comparative example shown in Fig. 6 (a) and the test example shown in Fig. 7 (a).

As a result of measuring the Charpy absorbed energy while varying the temperature of the specimen, in a comparative example in which a notch 10 of a 'V' shape was formed, the soft-brittle transition temperature (DBTT) It can be seen that brittle fracture occurs at a temperature of -100 ° C or lower.

On the other hand, in the test example in which a plurality of fine cracks 20 were vertically processed in the V-shaped notch 10, even at a temperature of -140 DEG C, a high absorption energy of 100 J or more was exhibited, It can be seen that the energy has risen several tens of times. However, the effect of improving the toughness due to the increase of the absorbed energy is confirmed only at a low temperature, and there is no greater effect than that of the comparative example in a high temperature region.

9 is a wavefront photograph of a comparative example and a test example according to a temperature change.

As shown in Fig. 9, in the comparative example, ductile fracture occurred at room temperature, but brittle fracture occurred at a temperature of -100 캜 or lower.

On the other hand, in the case of the test example, a dimple showing soft fracture occurred at a temperature range of -140 ° C., and the toughness of the material rapidly increased from the deformed shape of the specimen after the Charpy impact test. However, brittle fracture occurred at around -200 ℃, which is the liquid nitrogen temperature.

10 is a schematic view showing a notch of an arc-shaped sectional shape according to another embodiment of the present invention.

According to another embodiment of the present invention, a notch 10 'having an arc-shaped sectional shape as shown in Fig. 10 can be formed. In this case, compared with the wedge-shaped notch 10 of the above-described embodiment in which the lower end is sharp, an effect of more stress dispersion at the lower end of the notch 10 'can be expected. Of course, when a crack is generated due to a large stress, the fine crack 20 'formed in the direction perpendicular to the boundary 12' disperses the stress again, thereby preventing the crack from proceeding and inducing the soft fracture do.

In this case, the inclined surface 11 'may be formed as a curved surface as shown in FIG. 10, and as another example, it may include a curved surface having an arc sectional shape and a plane vertically upward or inclined at an angle .

10, 10 ': notch 11, 11': inclined surface
12,12 ': boundary 20, 20': fine crack
30: rolled surface 40: adjacent surface

Claims (16)

Wherein at least one notch is formed on the surface of the notch and at least one fine crack extending from the lower end of one side of the notch to the lower end of the other side of the inclined face (11) Wherein the low temperature characteristic is improved.
The apparatus of claim 1,
Characterized in that the metal material is formed on at least one surface of the raw material which is processed by cold rolling or cold forging.
The apparatus of claim 1,
Wherein the metal material has a wedge-shaped or circular arc-shaped cross-sectional shape.
The apparatus of claim 1,
A pair of inclined surfaces, and a boundary portion in which the pair of inclined surfaces are in contact with each other.
The method according to claim 4, wherein the micro-
Wherein the metal material is processed in a direction perpendicular to the longitudinal direction of the boundary portion.
The method according to claim 1,
Wherein a plurality of metal particles are formed spaced apart from each other.
3. The method of claim 2,
Wherein the raw material is a metal having a body-centered cubic (BCC) crystal structure.
A notch forming step of forming a notch on the surface; And
And at least one fine crack is formed on the lower end of the notch so as to extend from the lower end of the one side slope (11) of the notch to the lower end of the other side slope (11) through the boundary (12) ≪ / RTI >
9. The method of claim 8,
Wherein the notch is formed in a wedge-shaped or circular arc-shaped cross-sectional shape.
9. The method of claim 8,
Further comprising a step of pushing down the raw material before the step of forming the notch.
11. The method of claim 10,
Wherein the raw material is processed by cold rolling or cold forging.
9. The method of claim 8,
Wherein the notch includes a pair of inclined surfaces and a boundary portion where the pair of inclined surfaces are in contact with each other and the fine cracks are processed in a direction perpendicular to the longitudinal direction of the boundary portion. Way.
9. The method of claim 8,
Wherein the notch is formed by pressing, cutting, discharging, or laser processing.
9. The method of claim 8,
Wherein the fine cracks are formed by cutting, discharging, or laser processing.
9. The method of claim 8,
Wherein a plurality of micro cracks are formed spaced apart from each other.
12. The method of claim 11,
Wherein the raw material is made of a metal having a body-centered cubic (BCC) crystal structure.

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