KR101461736B1 - Austenitic steel having excellent machinability and superior cryogenic toughness in weld heat-affected zone and manufacturing method thereof - Google Patents

Abstract

(Excluding 0%) of carbon (C) and copper (Cu) satisfying manganese (Mn) of 15 to 35%, 23.6C + Mn? 28 and 33.5C- (Cr) (excluding 0%), sulfur (S): 0.03 to 0.1%, calcium (Ca): 0.001 to 0.01%, the balance iron (Fe) and other unavoidable impurities satisfying 28.5C + 4.4Cr? And a Charpy impact value at -196 캜 of the weld heat affected zone of 41 J or more, and excellent austenitic steels having excellent cryogenic and welding heat-affected cryogenic toughness, and a process for producing the same.
According to the present invention, a low-cost cryogenic steel material can be obtained, a stable austenite phase can be formed at a low temperature, carbide formation can be effectively suppressed, and a structural steel material excellent in machinability and welding heat impact cryogenic temperature toughness can be provided.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an austenitic steels excellent in machinability and machinability,

The present invention relates to austenitic steels excellent in machinability and weld heat affected zone cryogenic toughness and a method for producing the same. And more particularly to a structural steel used for a wide range of temperatures from a low temperature to a room temperature in a liquefied gas atmosphere such as a liquefied gas storage tank and a transportation facility.

Liquefied gases such as liquefied natural gas (LNG), liquid oxygen (Liquefied Oxygen, boiling point: -183 ° C), liquid nitrogen (Liquefied Nitrogen, boiling point: -196 ° C) in need. Therefore, in order to store these gases, a structure such as a pressure vessel made of a material having sufficient toughness and strength at a cryogenic temperature is required.

As a material usable at a low temperature in a liquefied gas atmosphere, Cr-Ni-based stainless alloy such as AISI 304, 9% Ni steel, and 5000-series aluminum alloy have been used. However, in the case of aluminum alloy, there is a problem that the design thickness of the structure is increased due to high alloy cost and low strength, and the use of the alloy is limited due to poor weldability. Cr-Ni-based stainless steel and 9% Ni steel have overcome the problems of the physical properties of aluminum, but they have been problematic in application because of high nickel content and high manufacturing cost.

In order to solve this problem, Patent Document 1 and Patent Document 2 are listed as technologies in which nickel content of high price is reduced and manganese and chromium are added instead. Patent Document 1 is a technique in which nickel content is reduced to 1.5 to 4% and manganese and chromium are respectively added to 16 to 22% and 2 to 5.5% to secure austenite structure to improve cryogenic toughness. Patent Document 2 discloses a nickel content Is reduced to about 5.5%, and manganese and chromium are added to 2.0% and 1.5%, respectively, to refine the ferrite crystal grains through repeated heat treatment and tempering to ensure extremely low temperature toughness. However, the above-mentioned inventions still contain expensive nickel and are subjected to repeated heat treatment and tempering in order to secure cryogenic toughness, which is not advantageous in terms of cost and simplification of the process.

Another technology related to structural molten steel used for liquefied gas is so-called nickel-free high manganese steel completely excluded from nickel. The high manganese steel is divided into ferrite and austenite depending on the amount of manganese added. For example, Patent Document 3 discloses a technique in which 5% manganese is added instead of 9% nickel and the crystal grains are finely refined through four times of heat treatment in an abnormal temperature range where austenite and ferrite coexist and then tempered to improve cryogenic toughness to be. Also, in Patent Document 4, 13% manganese is added to refine the crystal grains through four repeated heat treatments in the aberrant temperature range of austenite and ferrite, and then tempering to improve cryogenic toughness. These patents are mainly made up of ferrite as a main structure, and refining the ferrite grains through refining and tempering four or more times to obtain cryogenic toughness. However, these techniques have problems in that the cost increases and the load on the heat treatment equipment is increased due to the increase in the heat treatment number.

Patent Document 5 discloses a technique for a high manganese steel excellent in cryogenic properties by adding a large amount of manganese, 16 to 35%, and 0.1 to 0.5% carbon to stabilize austenite and adding 1 to 8% of aluminum, instead of completely eliminating nickel Patent Document 6 reports that a high manganese having excellent low-temperature toughness can be obtained by obtaining a mixed structure of austenite and eucullen martensite through addition of 15 to 40% manganese. However, since the content of carbon is low, there is a fear of deterioration of toughness due to formation of unstable structure at a cryogenic temperature such as a part of run-in martensite, and the possibility of occurrence of casting defects due to aluminum addition is increased.

In addition, the austenitic high manganese steels are subject to machining degradation due to high work hardening, which reduces the tool life and reduces the production costs, such as increased tool cost and increased downtime associated with tool replacement.

Korean Patent Publication No. 1998-0058369 International Publication No. WO 2007/080646 United States Patent No. 4257808 Korean Patent Publication No. 1997-0043149 Korean Patent Publication No. 1991-0012277 Japanese Laid-Open Patent No. 2007-126715

One aspect of the present invention suggests a low cost steel material excluding nickel and austenitic steel material which forms a stable austenite phase at a low temperature and has excellent machinability and secures cryogenic toughness at the weld heat affected zone and a manufacturing method thereof I want to.

However, the problems to be solved by the present invention are not limited to the above-mentioned problems, and other problems not mentioned can be clearly understood by those skilled in the art from the following description.

In order to achieve the above object, an aspect of the present invention is to provide carbon (C) having a content of manganese (Mn) of 15-35%, 23.6C + Mn? 28 and 33.5C- , Chrome (Cr) (excluding 0%), iron (Fe) and other unavoidable impurities satisfying 5% or less (excluding 0%) and 28.5C + 4.4Cr? Austenitic steels excellent in machinability and welding heat-affected area cryogenic toughness are provided.

Another aspect of the present invention is to provide a carbon steel sheet which comprises 15 to 35% of manganese (Mn), 5% or less of carbon (C) satisfying 23.6C + Mn? 28 and 33.5C- (Excluding 0%), chromium (Cr) (excluding 0%) satisfying 28.5C + 4.4Cr? 57, sulfur (S): 0.03-0.1%, calcium (Ca) (Fe) and other unavoidable impurities to obtain a weld heat affected zone; And cooling the weld heat affected zone at a cooling rate of 10 ° C / s or higher. The present invention also provides a method of manufacturing a wear-resistant austenitic steels excellent in machinability and weld heat-affected zone toughness.

According to the present invention, the content of manganese can be controlled to a relatively low range without adding nickel and aluminum, so that a low-cost cryogenic steel material can be obtained, a stable austenite phase is formed at a low temperature, It is possible to provide a structural steel excellent in cryogenic temperature due to welding heat affected by improving the machinability through addition of calcium and sulfur.

1 is a graph showing a relationship between manganese and carbon content according to an embodiment of the present invention.
2 is a photograph of a room temperature optical microstructure of a steel material according to an embodiment of the present invention.
3 is a wave front view of a steel material according to an embodiment of the present invention after a Charpy impact test at -196 캜.
4 is a graph showing the relationship between sulfur content and machinability according to an embodiment of the present invention.

Hereinafter, the austenitic steels excellent in machinability and weld heat affected zone cryogenic toughness of the present invention and a method for producing the same will be described in detail so that those skilled in the art can easily carry out the present invention.

Welding Heat Affected Zone When the content of manganese is low in austenitic steels requiring cryogenic toughness, the content of carbon must be increased in order to stabilize austenite, which may deteriorate toughness due to formation of carbides. In order to secure cryogenic toughness, it is necessary to suppress the precipitation of carbide. In the case where the control of the cooling rate is not easy as in the case of the weld heat affected zone, carbide precipitation occurs in the heat affected zone, which causes the cryogenic toughness to deteriorate rapidly. Therefore, it is urgently required to develop a cryogenic steel material excellent in cryogenic temperature toughness due to welding heat affected by stabilizing austenite through proper control of manganese and carbon and adding an alloying element effective for forming carbide to manganese. In addition, it is necessary to develop a low cost cryogenic steel material that does not contain expensive nickel or the like.

The inventors of the present invention have found that, in order to obtain a toughness at an extremely low temperature, a steel material to which nickel is not added needs to adjust the composition of the steel material and to make the main structure of the steel material austenite. Particularly, Generation of the protein is required to be controlled, leading to the present invention. In addition, the composition of steel which can significantly improve the machinability of austenitic high manganese steel has been derived by controlling the contents of calcium and sulfur.

Accordingly, the steel material of the present invention contains, by weight%, carbon (C) satisfying Mn: 15 to 35%, 23.6C + Mn? 28 and 33.5C-Mn? 0% is excluded), chromium (Cr) (excluding 0%), sulfur (S): 0.03 to 0.1%, calcium (Ca): 0.001 to 0.01%, and the balance iron Fe) and other unavoidable impurities. Here, Mn, C, and Cr in the respective formulas indicate the content of the corresponding element

The reason for limiting the numerical values of the above components will be described as follows. Hereinafter, it is necessary to pay attention that the content unit of each component is weight% unless otherwise stated.

manganese( Mn ): 15 to 35%

Manganese is an important element added to the high manganese steel such as the present invention, and is an element that stabilizes austenite. It is preferable that the amount of the austenite phase is 15% or more in order to stabilize the austenite phase at a cryogenic temperature. When the content of manganese is less than 15%, tough run martensite, which is a metastable phase, is formed when the carbon content is small and toughness can not be ensured because it transforms easily into alpha martensite due to processing organic transformation at an extremely low temperature. Further, in order to prevent this, when the austenite is stabilized by increasing the carbon content, the physical properties are deteriorated rapidly due to the precipitation of carbide, which is not preferable. Therefore, the content of manganese is preferably 15% or more. On the other hand, when the content of manganese exceeds 35%, the corrosion rate of the steel is lowered and the economical efficiency is decreased due to the increase of the content. Therefore, the content of manganese is preferably limited to 15 to 35%.

Carbon (C): 23.6C + Mn Gt; 28 < / RTI > and 33.5C- Mn? 23 Satisfies the relationship of

Carbon is an element that stabilizes and strengthens austenite, and it plays a role in lowering M _s and M _d , which are transformation points from austenite to bismuth or alpha martensite during cooling or processing. Therefore, when carbon is insufficiently added, the austenite is not stable enough to obtain a stable austenite at a cryogenic temperature. Further, it can be easily processed by external stress to cause machining organic transformation to alpha-martensite to reduce toughness, Reduces the strength. On the contrary, when the carbon content is excessive, the toughness is rapidly deteriorated due to the precipitation of carbide, and the workability is deteriorated due to an excessive increase of the strength.

Particularly, in the present invention, it is preferable to determine the content of carbon in consideration of the relationship with carbon and other elements to be added together. For this purpose, the relationship between carbon and manganese for formation of carbide found by the present inventors is shown in FIG. . Carbides are formed by carbon, but carbon does not independently affect the formation of carbides, but rather acts in a complex manner with manganese, affecting the formation tendency. 1 shows the optimum carbon content. In order to prevent formation of carbide, it is necessary to control the value of 23.6C + Mn (C and Mn represent the content of each component in terms of% by weight) to 28 or more, on condition that the other components satisfy the range defined in the present invention desirable. This means the oblique left boundary of the parallelogram region of the drawing. When 23.6C + Mn is less than 28, the stability of the austenite is decreased, and the processed organic transformation is caused by the impact at the cryogenic temperature, and the impact toughness is lowered. When the carbon content is too high, that is, when 33.5C-Mn is larger than 23, carbide precipitates due to excessive addition of carbon, which lowers impact toughness at low temperatures. Consequently, in the present invention, manganese is preferably added so as to satisfy 15-35, 23.6C + Mn? 28 and 33.5C-Mn? 23. As can be seen from Fig. 1, the lowest C content is 0% within the range satisfying the above formula.

Copper( Cu ): Less than 5% (0% excluded)

Copper has a very low solubility in the carbide and is slow to diffuse in the austenite so that it is concentrated at the interface of the austenite and the nucleated carbide, thereby inhibiting the diffusion of carbon, effectively slowing the growth of carbide, . In the case of the base material, precipitation of carbide can be suppressed through accelerated cooling during the manufacturing process, but since the cooling rate control is not easy in the heat affected zone of the weld, copper is added as an element which is very effective in inhibiting carbide precipitation. Copper also has the effect of stabilizing austenite and improving cryogenic toughness. However, when the content of Cu exceeds 5%, the hot workability of the steel material is lowered. Therefore, the upper limit is preferably limited to 5%. Further, the content of copper for obtaining the above-described carbide-suppressing effect is more preferably 0.5% or more.

chrome( Cr ): 28.5C + 4.4 Cr? 57  (Excluding 0%)

Chromium stabilizes the austenite up to the appropriate amount of added amount to improve impact toughness at low temperatures and solidifies in the austenite to increase the strength of the steel. Chromium is also an element that improves the corrosion resistance of steel. However, chromium is a carbide element, and it is also an element that reduces carbothermal effects at austenitic grain boundaries to reduce cold shock. Therefore, it is preferable that the content of chromium added in the present invention is determined in consideration of the relationship between carbon and other elements to be added together. In order to prevent formation of carbide, it is assumed that the other components satisfy the range defined in the present invention It is preferable to control the value of 28.5C + 4.4Cr (C and Cr indicate the content of each component in weight%) to 57 or less. When the value of 28.5C + 4.4Cr is more than 57, it is difficult to effectively suppress the formation of carbide in the austenite grain boundary due to excessive chromium and carbon content, and thus the impact toughness at low temperature is reduced. Therefore, in the present invention, it is preferable that chromium is added so as to satisfy 28.5C + 4.4Cr ≤ 57.

Sulfur (S): 0.03 to 0.1%

Sulfur is generally added together with manganese to form manganese sulfide as a compound, which is known to be an element that is easily cut and separated during cutting to improve cutting performance. It is possible to reduce the frictional force between the chip and the cutting tool, thereby reducing the wear of the cutting tool through lubrication of the tool surface, and preventing the cutting edge on the cutting tool, thereby increasing the life of the cutting tool. However, in the case of an excessive amount of sulfur, the large amount of coarse manganese sulfide stretched during hot working can reduce the mechanical properties of the steel and may impair the hot workability due to the formation of iron sulfide. Therefore, the upper limit is preferably 0.1% and less than 0.03% It is preferable to limit the lower limit to 0.03%.

calcium( Ca ): 0.001 to 0.01%

Calcium is an element commonly used to control the shape of manganese sulfide. Since it has a large affinity for sulfur, it forms calcium sulphide and is dissolved in manganese sulphide. Since the manganese sulphide is crystallized by using this calcium sulphide as nuclei, it inhibits the elongation of manganese sulphide during hot working, Thereby improving machinability. However, when the content is more than 0.01%, the effect is saturated and the calcium content is low. Therefore, in order to increase the content, it is necessary to add a large amount, which is not preferable from the viewpoint of the production cost. When the content is less than 0.001%, the effect is insignificant. %. ≪ / RTI >

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

The steel material of the present invention may contain, in addition to the above components, 0.5% or less of titanium (Ti), 0.5% or less of niobium (Nb), 0.5% or less of vanadium (V) % ≪ / RTI >

titanium( Ti ): Not more than 0.5%

Titanium is an element that increases the strength through employment and precipitation hardening effect. Especially, it is an advantageous element that can prevent the grain growth from being deteriorated due to titanium carbide / nitride in the weld heat affected zone and prevent deterioration of strength. However, when it is added in an amount exceeding 0.5%, coarse precipitates are formed and deteriorate the physical properties of the steel, so that the upper limit of the content is preferably limited to 0.5%.

Niobium ( Nb ): Not more than 0.5%

Niobium is an element that increases the strength through precipitation hardening effect. In particular, it can increase the recrystallization stop temperature (Tnr) of the steel to improve the yield strength through grain refinement at low temperature rolling. However, when it is added in excess of 0.5% Coarse precipitates are produced and rather deteriorate the physical properties of the steel. Therefore, the upper limit of the content thereof is preferably limited to 0.5%.

Vanadium (V): not more than 0.5%

Vanadium is an element which increases the strength through the effect of precipitation hardening, and when it is added in excess of 0.5%, coarse precipitate is formed, which deteriorates the physical properties of the steel. Therefore, the upper limit of the content is preferably limited to 0.5%.

Nitrogen (N): 1% or less

Nitrogen is an element that stabilizes austenite in addition to carbon, and is an especially advantageous element for enhancing strength through solid solution strengthening such as carbon. However, if it exceeds 1%, a coarse nitride is formed, which deteriorates the physical properties of the steel. Therefore, the content of nitrogen is preferably limited to 1% or less.

The steel having the above composition has an austenitic structure and is excellent in the cryogenic temperature toughness of the weld heat affected zone. According to a preferred embodiment of the present invention, the steel material of the present invention may have a Charpy impact value at -196 캜 of the welding heat affected zone of 41 J or more.

The steel of the present invention can be manufactured through a hot rolling and cooling process within the range of the composition satisfying the above-mentioned component system, or can be manufactured through a reheating and cooling process after hot rolling, and the microstructure of the weld heat affected zone is austenite It is preferable that the proportion is 95% or more. In addition to the austenite, some of the inevitably formed impurity such as martensite, bainite, pearlite, ferrite and the like may be partially included. Here, it is necessary to note that the content of each texture does not include precipitates such as carbides but is the content when the sum of the phases of the steel is regarded as 100%. In the steel material of the present invention, the microstructure of the weld heat affected zone preferably has a carbide content in the austenite grain boundary of not more than 5% by area.

The cryogenic temperature as a criterion for determining the fraction of the austenite structure is set at -196 ° C. That is, when the austenite structure fraction is out of the above range, sufficient toughness, that is, impact toughness of 41 J or more at -196 캜 can not be obtained.

Hereinafter, a method for producing the austenitic steels excellent in cryogenic temperature toughness at the weld heat affected zone of the present invention will be described.

In one embodiment, a slab satisfying the above-mentioned composition is welded to a welded heat affected zone by steel sheet (steel) produced by rough rolling, finishing and subsequent cooling. Then, the weld heat affected zone is cooled to a cooling rate of 10 DEG C / s or more.

The reason why the cooling rate of the weld heat affected zone is set to 10 ° C / s or more is to obtain a structure in which the austenite area fraction in the weld heat affected zone is 95% or more and the carbide existing in the austenite grain boundary is 5% It is for this reason. That is, the above-mentioned cooling rate is a cooling rate favorable for suppressing carbide formation when the amount of addition of Cr and C, which are carbide forming elements, is large.

Hereinafter, the present invention will be described in detail with reference to Examples. However, the following examples are only for illustrating the present invention in more detail and do not limit the scope of the present invention.

[ Example ]

Steel slabs were produced using continuous casting as the inventive and comparative examples satisfying the component systems described in Table 1 below.

The steel slab thus produced was heated under the conditions shown in Table 2, subjected to hot rolling and cooling, and the weld heat affected zone was cooled.

division Heating furnace temperature (캜) Rolling end temperature (캜) Cooling rate (° C / s) Cooling speed of welding part (℃ / s) Inventory 1 1160 912 16.7 17 Inventory 2 1160 875 18.6 25 Inventory 3 1150 880 18.7 28 Honorable 4 1150 878 19.2 26 Inventory 5 1140 896 25.4 12 Comparative Example 1 1160 920 5.2 18 Comparative Example 2 1160 925 11.5 25 Comparative Example 3 1140 895 15.3 28 Comparative Example 4 1180 887 3.54 12 Comparative Example 5 1180 932 3.62 12

The carbide area fraction, yield strength, tensile strength, elongation and Charpy impact value at -196 캜 of the weld heat affected zone of the welded heat affected zone were measured and shown in Table 3 below. For the machinability evaluation, drill holes are repeatedly drilled in a steel material at a rotating speed of 130 rpm and a drill advancing speed of 0.08 mm / rev using a high-speed tool steel drill having a diameter of 10 mm to measure the number of holes until the drill is worn down Are shown in Table 3.

division Area fraction of carbide in welded heat affected zone (%) Yield strength
(MPa) The tensile strength
(MPa) Elongation
(%) Weld heat affected part -196 ℃ Charpy impact value (J) Number of holes
(dog) Inventory 1 0.2 405 1012 56 65 8 Inventory 2 0.3 467 896 45 135 12 Inventory 3 0.3 470 898 44 132 14 Honorable 4 0.3 472 890 46 125 17 Inventory 5 One 442 1013 56 116 8 Comparative Example 1 0.8 323 1006 70 62 2 Comparative Example 2 0.8 356 972 58 73 One Comparative Example 3 0.6 432 984 48 131 3 Comparative Example 4 0 385 865 54 125 4 Comparative Example 5 0 365 845 57 134 4

In the case of this embodiment, the addition of copper is effective in suppressing precipitation of intergranular carbides in the weld heat affected zone as a steel grade which satisfies both the composition system and the composition range controlled by the present invention in the content of carbon, manganese, chromium and copper, It can be seen that the low-temperature toughness is excellent as the fraction is controlled to 5% or less. Concretely, even at a high carbon content, the carbide was effectively suppressed by the addition of copper, so that the desired microstructure and physical properties were obtained.

In Comparative Examples 1 to 5, it can be known that the machinability is such that the content of calcium and sulfur does not satisfy the composition range controlled by the present invention, and the machinability is inferior.

On the other hand, Examples 1 to 5 show that the addition amount of sulfur and calcium satisfies both the composition and the composition range controlled by the present invention and is superior to the comparative example in machinability. Particularly, Examples 2 to 4 show that when the sulfur content is changed, the machinability is further improved by increasing the sulfur content.

FIG. 2 is a microstructure photograph of a steel sheet produced according to the third embodiment of the present invention. It can be seen that all the structures are composed of carbides of 5% or less in terms of austenite and area percentage. FIG. 3 is a cross- , Showing the shape of ductile fracture, and it can be confirmed that the austenite stabilization was effectively possible by controlling the composition system and the composition range of the present invention. 4 shows the machinability according to the sulfur content. It can be seen that the machinability increases with increasing sulfur content.

Claims (6)

(Excluding 0%) of carbon (C) and copper (Cu) satisfying manganese (Mn) of 15 to 35%, 23.6C + Mn? 28 and 33.5C- (Cr) (excluding 0%), sulfur (S): 0.03 to 0.1%, calcium (Ca): 0.001 to 0.01%, the balance iron (Fe) and other unavoidable impurities satisfying 28.5C + 4.4Cr? , And a Charpy impact value at -196 캜 of the weld heat affected zone of not less than 41 J. Austenitic steels excellent in machinability and weld heat-affected cryogenic toughness. The method according to claim 1,
Wherein the steel material further comprises at least one of titanium (Ti): not more than 0.5%, niobium (Nb): not more than 0.5%, vanadium (V): not more than 0.5%, nitrogen (N): not more than 1% Austenitic steels excellent in cryogenic and weld heat affected zone cryogenic toughness. The method according to claim 1,
Wherein the microstructure of the weld heat affected zone is austenite having an area fraction of 95% or more. The method according to claim 1,
Austenitic steels excellent in machinability and cryogenic heat cryogenic temperature toughness, wherein the carbide present in the austenitic grain boundaries of the weld heat affected zone has an area fraction of 5% or less. (Excluding 0%) of carbon (C) and copper (Cu) satisfying manganese (Mn) of 15 to 35%, 23.6C + Mn? 28 and 33.5C- (Cr) (excluding 0%), sulfur (S): 0.03 to 0.1%, calcium (Ca): 0.001 to 0.01%, the balance iron (Fe) and other unavoidable impurities satisfying 28.5C + 4.4Cr? To obtain a weld heat affected zone; And
And cooling the weld heat affected zone at a cooling rate of 10 ° C / s or higher. 6. The method of claim 5,
Wherein the steel material further comprises at least one of titanium (Ti): not more than 0.5%, niobium (Nb): not more than 0.5%, vanadium (V): not more than 0.5%, nitrogen (N): not more than 1% Wherein the cryogenic and weld heat affected zone is characterized by excellent cryogenic toughness.

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