EP2519654B1 - Ultra-high-strength steel wire having excellent resistance to delayed fracture and manufacturing method thereof - Google Patents
Ultra-high-strength steel wire having excellent resistance to delayed fracture and manufacturing method thereof Download PDFInfo
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- EP2519654B1 EP2519654B1 EP10841192.7A EP10841192A EP2519654B1 EP 2519654 B1 EP2519654 B1 EP 2519654B1 EP 10841192 A EP10841192 A EP 10841192A EP 2519654 B1 EP2519654 B1 EP 2519654B1
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- steel
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- wire rod
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- precipitates
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- 238000004519 manufacturing process Methods 0.000 title claims description 20
- 230000003111 delayed effect Effects 0.000 title claims description 13
- 229910000797 Ultra-high-strength steel Inorganic materials 0.000 title claims description 7
- 229910000831 Steel Inorganic materials 0.000 claims description 82
- 239000010959 steel Substances 0.000 claims description 82
- 239000002244 precipitate Substances 0.000 claims description 33
- 229910001562 pearlite Inorganic materials 0.000 claims description 27
- 229910052739 hydrogen Inorganic materials 0.000 claims description 25
- 239000001257 hydrogen Substances 0.000 claims description 25
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 claims description 24
- 238000000034 method Methods 0.000 claims description 21
- 229910052757 nitrogen Inorganic materials 0.000 claims description 19
- 229910000859 α-Fe Inorganic materials 0.000 claims description 10
- 238000001816 cooling Methods 0.000 claims description 9
- 229910052720 vanadium Inorganic materials 0.000 claims description 7
- 229910052799 carbon Inorganic materials 0.000 claims description 6
- 239000012535 impurity Substances 0.000 claims description 6
- 230000009467 reduction Effects 0.000 claims description 6
- 238000010622 cold drawing Methods 0.000 claims description 5
- 238000010438 heat treatment Methods 0.000 claims description 5
- 229910052748 manganese Inorganic materials 0.000 claims description 5
- 229910052710 silicon Inorganic materials 0.000 claims description 5
- 238000005098 hot rolling Methods 0.000 claims description 4
- 239000002245 particle Substances 0.000 claims description 2
- 230000000052 comparative effect Effects 0.000 description 20
- 239000011572 manganese Substances 0.000 description 9
- 230000008569 process Effects 0.000 description 9
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 8
- 230000000694 effects Effects 0.000 description 8
- 238000005275 alloying Methods 0.000 description 7
- 230000007423 decrease Effects 0.000 description 6
- 238000005096 rolling process Methods 0.000 description 6
- 229910001567 cementite Inorganic materials 0.000 description 5
- 239000011159 matrix material Substances 0.000 description 5
- 229910000742 Microalloyed steel Inorganic materials 0.000 description 4
- 230000015572 biosynthetic process Effects 0.000 description 4
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 4
- 229910000851 Alloy steel Inorganic materials 0.000 description 3
- 230000008859 change Effects 0.000 description 3
- 229910052804 chromium Inorganic materials 0.000 description 3
- 238000010586 diagram Methods 0.000 description 3
- 238000005516 engineering process Methods 0.000 description 3
- 239000000203 mixture Substances 0.000 description 3
- 238000004881 precipitation hardening Methods 0.000 description 3
- 230000003746 surface roughness Effects 0.000 description 3
- LEONUFNNVUYDNQ-UHFFFAOYSA-N vanadium atom Chemical compound [V] LEONUFNNVUYDNQ-UHFFFAOYSA-N 0.000 description 3
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 2
- 229910001566 austenite Inorganic materials 0.000 description 2
- 238000011161 development Methods 0.000 description 2
- 238000005242 forging Methods 0.000 description 2
- 229910000734 martensite Inorganic materials 0.000 description 2
- 238000005259 measurement Methods 0.000 description 2
- 229910052750 molybdenum Inorganic materials 0.000 description 2
- 229910052758 niobium Inorganic materials 0.000 description 2
- 238000001556 precipitation Methods 0.000 description 2
- 238000007670 refining Methods 0.000 description 2
- 238000011160 research Methods 0.000 description 2
- 238000005482 strain hardening Methods 0.000 description 2
- 238000005496 tempering Methods 0.000 description 2
- 229910052719 titanium Inorganic materials 0.000 description 2
- 230000009466 transformation Effects 0.000 description 2
- 238000005491 wire drawing Methods 0.000 description 2
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 description 1
- OAICVXFJPJFONN-UHFFFAOYSA-N Phosphorus Chemical compound [P] OAICVXFJPJFONN-UHFFFAOYSA-N 0.000 description 1
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 description 1
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 description 1
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
- 238000000137 annealing Methods 0.000 description 1
- 238000003776 cleavage reaction Methods 0.000 description 1
- 238000010273 cold forging Methods 0.000 description 1
- 229910052802 copper Inorganic materials 0.000 description 1
- 238000005260 corrosion Methods 0.000 description 1
- 230000007797 corrosion Effects 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 150000002431 hydrogen Chemical class 0.000 description 1
- 229910052759 nickel Inorganic materials 0.000 description 1
- 229910052698 phosphorus Inorganic materials 0.000 description 1
- 239000011574 phosphorus Substances 0.000 description 1
- 238000010791 quenching Methods 0.000 description 1
- 230000000171 quenching effect Effects 0.000 description 1
- 230000003014 reinforcing effect Effects 0.000 description 1
- 230000007017 scission Effects 0.000 description 1
- 239000010703 silicon Substances 0.000 description 1
- 238000004513 sizing Methods 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
- 239000006104 solid solution Substances 0.000 description 1
- 239000000243 solution Substances 0.000 description 1
- 229910052717 sulfur Inorganic materials 0.000 description 1
- 239000011593 sulfur Substances 0.000 description 1
- 230000001131 transforming effect Effects 0.000 description 1
- 238000009827 uniform distribution Methods 0.000 description 1
Images
Classifications
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
- C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/0093—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for screws; for bolts
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
Definitions
- the present invention relates to a steel wire rod which is used for the manufacturing of automotive engine bolts requiring ultra-high strength, and more particularly to a steel wire rod having excellent resistance to hydrogen delayed-fracture and a manufacturing method thereof.
- high-strength bolts are being manufactured as bolts having a strength of about 1200 MPa from an alloy steel such as SCM435 or SCM440 through quenching and tempering.
- a steel having a tensile strength of 1300 MPa is likely to undergo delayed fracture by hydrogen, and thus has not been used for the manufacturing of super-high-strength bolts.
- delayed fracture refers to a phenomenon in which bolts suddenly fracture when a specific tensile strength (about 1200 MPa) is applied thereto. This phenomenon occurs mainly at the notch or head portions of bolts and is known to be attributable to hydrogen embrittlement in a triaxial stress state.
- tensile strength about 1200 MPa
- a Japanese steel company has developed a high-strength pearlite steel based on pearlite, which has improved resistance to delayed fracture through hydrogen trap sites formed at the pearlite/cementite and maintains the characteristic strength of pearlite.
- This pearlite steel is being supplied to some automobile companies.
- this pearlite steel has the disadvantages of a high production cost and a complex manufacturing process. Another disadvantage is that very accurate cooling conditions are required for the production of the steel.
- US2003079815 A1 teaches a high strength, perlitic steel wire with superior drawability.
- microalloyed steel for substituting for the bolt alloy steel that has been used to date in automobile engines has been much studied in terms of reducing the production cost by omitting heat treatment.
- complex forging designs have been used in order to impart lightweight and high-strength properties to automobiles and reduce the number of automobile parts, and such complex forging designs may cause deformation in the microalloyed steel when a conventional annealing and tempering process is applied. For this reason, it is substantially impossible to apply the microalloyed steel.
- An aspect of the present invention provides a steel wire rod having both ultra-high strength and excellent resistance to delayed fracture, and a manufacturing method thereof.
- an ultra-high-strength steel wire rod having excellent resistance to delayed fracture, the wire rod including, by wt%, 0.7-1.2% C, 0.25-0.5% Si, 0.5-0.8% Mn, 0.02-0.1% V, 0.02% or less P, 0.02% or less S, optionally 60ppm or less N and a balance of Fe and inevitable impurities.
- a method for manufacturing an ultra-high-strength steel wire rod having excellent resistance to delayed fracture including the steps of: heating a steel, which includes, by wt%, 0.7-1.2% C, 0.25-0.5% Si, 0.5-0.8% Mn, 0.02-0.1% V, 0.02% or less P, 0.02% or less S, optionally 60ppm or less N and a balance of Fe and inevitable impurities, to 1000-1100°C, and hot-rolling the heated steel at a temperature of 900 ⁇ 1000°C; cooling the rolled steel to 600 ⁇ 650°C to a rate of 5 ⁇ 10°C/s; and cold-drawing the cooled steel at a reduction ratio of 60-80%.
- the strength of pearlite can be increased due to the precipitation hardening effect according to the addition of V, and diffusible hydrogen-trap sites can be increased through the formation of V(C,N) precipitates, thereby the hydrogen delayed-fracture resistance of the wire rod.
- the wire rod of the present invention when used for the manufacturing of automobile bolts and the like, it can contribute to a decrease in weight and an increase in performance of automobiles.
- the manufacturing method of the present invention offers excellent price competitiveness by omitting lead patenting and expensive alloying elements, and can act as the basis of novel manufacturing methods having no limitation in process conditions.
- the inventors of the present invention have conducted many studies to solve the delayed-fracture problem that is the biggest issue in developing high-strength bolts for automobiles. As a result, the present inventors have found that, when vanadium carbonitride is formed in the ferrite matrix of pearlite by adding a trace amount of vanadium, it increases the strength of pearlite and acts as a diffusible hydrogen-trap site to improve hydrogen delayed-fracture resistance, thereby reaching the present invention.
- compositional amounts are hereinafter expressed as wt%).
- the most important alloying element in the wire rod of the present invention is vanadium (V).
- the wire rod of the present invention has a V content of 0.02 ⁇ 0.1%.
- V forms V(C,N) precipitates in the ferrite matrix. The precipitates increase the strength of pearlite and also act as diffusible hydrogen-trap sites.
- V content is less than 0.02%, the solid solubility with nitrogen and carbon will decrease, making it difficult to effectively form precipitates, and if the V content is more than 0.1%, the precipitation of V in the ferrite matrix will be excessive, and thus it will cause fractures in the steel during rolling and drawing and rapidly reduce the cold forgeability of the steel.
- the content of carbon (C) in the steel is preferably 0.7 ⁇ 1.2%.
- C is an essential alloying element that is generally added to ensure the strength of steel. If the content of C is less than 0.7%, it cannot ensure sufficient strength, thus making it impossible to ensure an ultra-high-strength steel. If the C content is more than 1.2%, it can cause cracks or fractures during rolling and drawing processes.
- the content of manganese (Mn) in the steel is preferably 0.5 ⁇ 0.8%.
- Mn is an alloying element that increases the strength of the steel and influences the impact properties of the steel. Also, it increases the rolling properties of the steel and reduces the embrittlement of the steel. If the content of Mn is less than 0.5%, the strength reinforcing effect will be insignificant, and if it is more than 0.8%, it will result in severe hardening. For this reason, the content of Mn is preferably limited to 0.5 ⁇ 0.8wt%.
- the content of silicon (Si) in the steel is preferably 0.025 ⁇ 0.5%. Si forms a solid solution in the ferrite of pearlite to increase the strength of the steel. If the content of Si is less than 0.25%, the effect of increasing the strength of the steel will be insufficient, and if it is more than 0.5%, it will increase the hardening of the steel during cold forging to reduce the toughness of the steel.
- the content of phosphorus (P) in the steel is preferably 0.02% or less. Because P can be segregated in the grain boundary to reduce the toughness of the steel, the content of P is preferably as low as possible. For this reason, the upper limit of the P content is preferably limited to 0.02%.
- the content of sulfur (S) in the steel is preferably 0.02% or less.
- S which is a low-boiling-point element, can bond with Mn to reduce the toughness of the steel and can also adversely affect the properties of the high-strength wire rod, and for this reason, the content thereof is preferably as low as possible.
- the upper limit of the S content is preferably limited to 0.02% in view of inevitable problems occurring in a refining process.
- N nitrogen
- the content of N is preferably not more than 60 ppm.
- the precipitate-firming elements Ti and Nb, other than V are not positively added, except for the case in which they are added as inevitable elements.
- Ti is added in combination with V
- nitrogen in the molten steel will first react with Ti to form a TiN precipitate, such that a V precipitate cannot be effectively formed, whereby the effect of improving the delayed-fracture resistance of the steel by the V precipitate cannot be obtained.
- V is added in combination with Nb
- the austenite grains can be refined, but the price of the steel will be inevitably increased, and Nb will interfere with the formation of a V precipitate, because it is highly reactive with nitrogen.
- the steel wire rod of the present invention comprises Fe and inevitable impurities.
- V(C,N) precipitates are preferably distributed in the ferrite structure of pearlite.
- the V(C,N) precipitates prevent the precipitation of film-like cementite and are distributed in the ferrite structure of pearlite to act as strong hydrogen-trap site, thereby improving the hydrogen delayed-fracture resistance of the steel.
- the average particle size of the V(C,N) precipitates is preferably 30 nm or less, and the number of the V(C,N) precipitates is accoding to the invention 1x10 9 /mm 2 or more.
- the size of the V(C,N) precipitates is more than 30 nm, these precipitates will not be finely distributed in the ferrite matrix of pearlite, and thus the effect of increasing the strength of the steel through the uniform distribution of the precipitates will not be obtained.
- the V(C,N) precipitates are coarse, they can form coarse precipitates in ferrite to cause fractures, rather than improving the tensile strength of the steel by suppressing the movement of dislocations.
- the size of the precipitates is preferably 30nm or less.
- the reason for which the number of the precipitates must be 1x10 9 /mm 2 or more is that, if the number of the precipitates is less than 1x10 9 /mm 2 , it will be difficult to ensure a precipitation hardening effect by the V precipitate, and thus the strength sought in the present invention cannot be achieved. If the number of the precipitates is too large, the precipitation hardening effect can be maximized to cause wire breakage during wire drawing.
- the steel wire rod of the present invention has a pearlite structure. As the lamellar spacing of the pearlite structure decreases, the tensile strength and ductility of the wire rod increase.
- the lamellar spacing of the pearlite structure of the steel wire rod according to the present invention is 150 ⁇ 300 nm.
- the ductility and strength of pearlite depend on the lamellar spacing of the pearlite. Particularly, the yield strength of pearlite depends on the lamellar spacing thereof, and this can be expressed by the Hall-Petch relationship. Thus, the lamellar spacing needs to be maintained at a suitable level, because a decrease in the lamellar spacing leads to an increase in strength and ductility.
- the lamellar spacing is less than 150 nm, the strain hardening rate of the wire rod will be excessively increased to cause wire breakage during wire drawing.
- the lamellar spacing is more than 300 nm, shear failures, such as cleavage fractures, will be highly likely to occur, and it will be difficult to ensure the strength described below.
- the content of diffusible hydrogen in the wire rod of the present invention is limited to 0.6 ⁇ 0.9 ppm.
- the term content of diffusible hydrogen refers to the highest concentration at which steel can contain hydrogen.
- the content of diffusible hydrogen varies depending on a matrix structure. If the content of diffusible hydrogen in the steel of the present invention is less than 0.6 ppm, the effect of improving resistance to delayed fracture by hydrogen trapping cannot be obtained.
- the reason for which the content of diffusible hydrogen in the steel wire rod of the present invention is limited to 0.9 ppm is that it is not easy to ensure a diffusible hydrogen content of more than 9 ppm in pearlite steels containing V precipitates, as in the case of the present invention.
- the heating temperature is 1100°C or lower, and preferably 1000 ⁇ 1100°C.
- the heated steel is subjected to hot rolling.
- a process ranging from rough rolling to finish rolling is carried out at a temperature of 900-1000°C.
- the rolled steel is cooled to 650 ⁇ 600°C at a rate of 5 ⁇ 10°C/s. If the cooling rate is less than 5°C, proeutectoid cementite will be precipitated to cause anisotropy, and if the cooling rate is more than 10°C/s, martensite, a low-temperature structure, will be formed.
- the steel cooled after hot rolling has a tensile strength of 1100 ⁇ 1300 MPa. After the cooling process, the steel is subjected to cold drawing.
- the cold drawing is carried out at a reduction ratio of 60 ⁇ 80%.
- the steel In order to ensure the tensile strength of the steel by work hardening through the cold drawing process, the steel is cold drawn at a reduction ratio of 60% or higher. If the reduction ratio is higher than 80%, the cold forgeability of the steel will be deteriorated. For this reason, the upper limit of the reduction ratio is preferably 80%.
- the cold-drawn wire rod has a tensile strength of 1550 ⁇ 1650 MPa.
- each of steels satisfying the compositions shown in Table 1 below was heated at 1100°C, after which a strain of 0.6 was applied thereto at a strain rate of 10/s at 950°C. Then, the steels were cooled at a rate of 7.5°C/s and drawn to 10 ⁇ 90%, thereby manufacturing wire rods.
- the inventive examples are steels to which V has been added within the content range specified in the present invention
- the conventional example is a steel to which Cr had been added.
- comparative examples 1 and 2 are steels that are out of the V content of the present invention
- comparative examples 3 and 4 are steels to which Al had been added in place of V.
- the wire rods manufactured as described above were measured for tensile strength, elongation and surface roughness, and the results of the measurement are shown in Table 2 below.
- the manufactured wire rods were measured for microstructure, fracture stress according to diffusible hydrogen content, and the change in tensile strength according to the amount of drawing, and the results of the measurement are shown in FIGS. 1 to 6 .
- inventive examples 1 and 2 could achieve excellent strength, elongation and surface roughness (RA), which were comparable to the conventional example and comparative examples 3 and 4, even when a trace amount of V was added thereto. Also, as can be seen in comparative examples 1 and 2, even when V was added in an amount higher than the upper limit of the content range specified in the present invention, strength and elongation were not improved. For this reason, the content of V was limited to 0.05 ⁇ 0.1% in view of diffusible hydrogen content and fracture stress.
- FIGS. 1A, 1B , 2A and 2B are photographs showing the results of observing the microstructures of the conventional example and inventive example 1, respectively.
- the lamellar spacing of pearlite in inventive example 1 (about 184.3 nm) was about 50% smaller than the conventional example (about 368.75 nm).
- FIGS. 3A through 3F show the results of observing the microstructures of the conventional example, inventive example 1, inventive example 2, comparative example 1, comparative example 2 and comparative example 4, respectively.
- inventive examples 1 and 2 had lamellar spacing gaps of 184.3 nm and 213 nm, respectively, which were smaller than the conventional example and the comparative examples.
- FIGS. 4A and 4B schematically show the microstructures of the conventional example and inventive example 1, respectively.
- film-like cementite was precipitated, but in inventive example 1, spherical V(C,N) precipitates were distributed.
- FIG. 5 shows the results of comparing hydrogen delayed-fracture resistance between inventive example 1 and the conventional example (82BC steel).
- inventive example 1 had a diffusible hydrogen content of 0.87 ppm which was about 1.5 times higher than the conventional example (diffusible hydrogen content: 0.52 ppm).
- the spherical V(C,N) precipitates shown in FIG. 4 were segregated in the ferrite-cementite of prior pearlite, whereby diffusible hydrogen was trapped by the spherical V(C,N) precipitates, thus improving the hydrogen delayed-fracture resistance of the wire rod.
- FIG. 6 is a graphic diagram showing a change in tensile strength according to a decrease in diameter in various drawing amounts. As can be seen in Table 2 above and FIG. 6 , the case in which V was added showed excellent tensile strength. If V is added in an amount of more than 0.1%, the increase in tensile strength caused by the formation of V precipitates can be expected, but in comparative examples 1 and 2 in which V was added in large amounts, the wire rods could be cracked or fractured due to the formation of excessive V precipitates.
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Description
- The present invention relates to a steel wire rod which is used for the manufacturing of automotive engine bolts requiring ultra-high strength, and more particularly to a steel wire rod having excellent resistance to hydrogen delayed-fracture and a manufacturing method thereof.
- In recent years, as the demand for lightweight and high-performance automobiles has increased, the high-strength requirement for engine parts such as bolts has increased, in order to reduce the consumption of energy. These days, high-strength bolts are being manufactured as bolts having a strength of about 1200 MPa from an alloy steel such as SCM435 or SCM440 through quenching and tempering. However, a steel having a tensile strength of 1300 MPa is likely to undergo delayed fracture by hydrogen, and thus has not been used for the manufacturing of super-high-strength bolts.
- The biggest issue to consider in the development of high-strength bolts is delayed fracture. The term delayed fracture refers to a phenomenon in which bolts suddenly fracture when a specific tensile strength (about 1200 MPa) is applied thereto. This phenomenon occurs mainly at the notch or head portions of bolts and is known to be attributable to hydrogen embrittlement in a triaxial stress state. Thus, in the development of high-strength bolts having a strength of about 1200 MPa or higher, it is required to ensure the safety of the bolts by increasing the resistance to delayed fracture thereof.
- A Japanese steel company has developed a high-strength pearlite steel based on pearlite, which has improved resistance to delayed fracture through hydrogen trap sites formed at the pearlite/cementite and maintains the characteristic strength of pearlite. This pearlite steel is being supplied to some automobile companies.
- However, in the above-mentioned pearlite steel, more than 0.2wt% Cr should be added during a drawing process for sizing after the production of the steel in order to improve the tensile strength and ensure the drawability thereof, and isothermal transformation is necessarily required. Thus, this pearlite steel has the disadvantages of a high production cost and a complex manufacturing process. Another disadvantage is that very accurate cooling conditions are required for the production of the steel.
- Also, in an attempt to improve the delayed-fracture resistance of high-strength steel wire rods having a strength of 1200 MPa or higher, there is a technique in which each of grain refining elements, including Ti, Nb and V, is added in an amount of 0.5wt% or higher, and in which corrosion-resistant elements, such as Ni, Cu, Co and the like, and carbide elements are added. However, this technique is disadvantageous in that the production cost of the steel is very high, because lead patenting is necessarily required to ensure the transformation stability of pearlite.
-
US2003079815 A1 teaches a high strength, perlitic steel wire with superior drawability. - Meanwhile, microalloyed steel for substituting for the bolt alloy steel that has been used to date in automobile engines has been much studied in terms of reducing the production cost by omitting heat treatment. However, in recent years, complex forging designs have been used in order to impart lightweight and high-strength properties to automobiles and reduce the number of automobile parts, and such complex forging designs may cause deformation in the microalloyed steel when a conventional annealing and tempering process is applied. For this reason, it is substantially impossible to apply the microalloyed steel.
- Thus, there has been research into a method of improving toughness through austenite grain refinement by reducing the content of C and adding a trace amount of Ti, and there has been research into a method of achieving high strength by forming acicular ferrite through the addition of a small amount of Mo. However, these methods have the problem of increasing production costs due to the addition of relatively expensive alloying elements.
- Also, there have been suggested methods of improving toughness by reducing the content of C to 0.1% and adding Cr and Mo and of transforming the microstructure of steel to martensite through controlled cooling. However, these methods have problems associated with a decrease in toughness, the addition of expensive elements such as Cr and Mo, and the embodiment of equipment for controlled cooling.
- Meanwhile, as mentioned above, limitations in further improving the tensile strength of an alloy steel having a tensile strength of about 1200 MPa have not yet been overcome. Also, although a few technologies related to ultra-high-strength wire rods were suggested in Japan, these technologies necessarily require the addition of expensive alloying elements and lead patenting, making it impossible to ensure price competitiveness. Particularly, it is actually difficult to ensure stable data on delayed-fracture characteristics caused by hydrogen.
- Accordingly, there has been a need for a technology of manufacturing ultra-high-strength steel wire rods, which reduces the number of necessary processes by omitting basic heat treatment, as in the case of microalloyed steel, ensures price competitiveness through the use of trace amounts of alloying elements and ensures resistance to delayed fracture.
- An aspect of the present invention provides a steel wire rod having both ultra-high strength and excellent resistance to delayed fracture, and a manufacturing method thereof.
- According to an aspect of the present invention, there is provided an ultra-high-strength steel wire rod having excellent resistance to delayed fracture, the wire rod including, by wt%, 0.7-1.2% C, 0.25-0.5% Si, 0.5-0.8% Mn, 0.02-0.1% V, 0.02% or less P, 0.02% or less S, optionally 60ppm or less N and a balance of Fe and inevitable impurities.
- According to another aspect of the present invention, there is provided a method for manufacturing an ultra-high-strength steel wire rod having excellent resistance to delayed fracture, the method including the steps of: heating a steel, which includes, by wt%, 0.7-1.2% C, 0.25-0.5% Si, 0.5-0.8% Mn, 0.02-0.1% V, 0.02% or less P, 0.02% or less S, optionally 60ppm or less N and a balance of Fe and inevitable impurities, to 1000-1100°C, and hot-rolling the heated steel at a temperature of 900∼1000°C; cooling the rolled steel to 600∼650°C to a rate of 5∼10°C/s; and cold-drawing the cooled steel at a reduction ratio of 60-80%.
- In the steel wire rod according to the present invention, the strength of pearlite can be increased due to the precipitation hardening effect according to the addition of V, and diffusible hydrogen-trap sites can be increased through the formation of V(C,N) precipitates, thereby the hydrogen delayed-fracture resistance of the wire rod. Thus, when the wire rod of the present invention is used for the manufacturing of automobile bolts and the like, it can contribute to a decrease in weight and an increase in performance of automobiles.
- Also, the manufacturing method of the present invention offers excellent price competitiveness by omitting lead patenting and expensive alloying elements, and can act as the basis of novel manufacturing methods having no limitation in process conditions.
- The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
-
FIGS. 1A and 1B are photographs showing the results of observing the microstructures of steel wire rods according to a conventional example and inventive example 1, respectively; -
FIGS. 2A and 2B are photographs showing the results of observing the microstructures of steel wire rods according to a conventional example and inventive example 1, respectively; -
FIGS. 3A through 3F are a set of photographs showing the results of observing the microstructures of steel wire rods according to a conventional example, inventive examples and comparative examples; -
FIGS. 4A and 4B schematically show the microstructures of steel wire rods according to a conventional example and inventive example 1, respectively; -
FIG. 5 is a graphic diagram showing the relationship between the fracture stress and diffusible hydrogen content of steel wire rods according to a conventional example and inventive example; and -
FIG. 6 is a graphic diagram showing the change in tensile strength according to diameter during drawing for steel wire rods of a conventional example, inventive examples and comparative examples. - Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. Like reference numerals in the drawings denote like elements, and thus their description will be omitted.
- The inventors of the present invention have conducted many studies to solve the delayed-fracture problem that is the biggest issue in developing high-strength bolts for automobiles. As a result, the present inventors have found that, when vanadium carbonitride is formed in the ferrite matrix of pearlite by adding a trace amount of vanadium, it increases the strength of pearlite and acts as a diffusible hydrogen-trap site to improve hydrogen delayed-fracture resistance, thereby reaching the present invention.
- Hereinafter, the steel wire rod of the present invention will be described in detail.
First, the composition of the steel wire rod of the present invention will now be described (compositional amounts are hereinafter expressed as wt%). - The most important alloying element in the wire rod of the present invention is vanadium (V). The wire rod of the present invention has a V content of 0.02∼0.1%. V forms V(C,N) precipitates in the ferrite matrix. The precipitates increase the strength of pearlite and also act as diffusible hydrogen-trap sites.
- If the V content is less than 0.02%, the solid solubility with nitrogen and carbon will decrease, making it difficult to effectively form precipitates, and if the V content is more than 0.1%, the precipitation of V in the ferrite matrix will be excessive, and thus it will cause fractures in the steel during rolling and drawing and rapidly reduce the cold forgeability of the steel.
- The content of carbon (C) in the steel is preferably 0.7∼1.2%. C is an essential alloying element that is generally added to ensure the strength of steel. If the content of C is less than 0.7%, it cannot ensure sufficient strength, thus making it impossible to ensure an ultra-high-strength steel. If the C content is more than 1.2%, it can cause cracks or fractures during rolling and drawing processes.
- The content of manganese (Mn) in the steel is preferably 0.5∼0.8%. Mn is an alloying element that increases the strength of the steel and influences the impact properties of the steel. Also, it increases the rolling properties of the steel and reduces the embrittlement of the steel. If the content of Mn is less than 0.5%, the strength reinforcing effect will be insignificant, and if it is more than 0.8%, it will result in severe hardening. For this reason, the content of Mn is preferably limited to 0.5∼0.8wt%.
- The content of silicon (Si) in the steel is preferably 0.025∼0.5%. Si forms a solid solution in the ferrite of pearlite to increase the strength of the steel. If the content of Si is less than 0.25%, the effect of increasing the strength of the steel will be insufficient, and if it is more than 0.5%, it will increase the hardening of the steel during cold forging to reduce the toughness of the steel.
- The content of phosphorus (P) in the steel is preferably 0.02% or less. Because P can be segregated in the grain boundary to reduce the toughness of the steel, the content of P is preferably as low as possible. For this reason, the upper limit of the P content is preferably limited to 0.02%.
- The content of sulfur (S) in the steel is preferably 0.02% or less. S, which is a low-boiling-point element, can bond with Mn to reduce the toughness of the steel and can also adversely affect the properties of the high-strength wire rod, and for this reason, the content thereof is preferably as low as possible. Thus, the upper limit of the S content is preferably limited to 0.02% in view of inevitable problems occurring in a refining process.
- In addition to the above-described elements, 60 ppm of nitrogen (N) may be added. N forms VN, but corresponds to an impurity that is incorporated into molten steel. For this reason, the content of N is preferably not more than 60 ppm.
- Meanwhile, in the present invention, the precipitate-firming elements Ti and Nb, other than V, are not positively added, except for the case in which they are added as inevitable elements. This is because, if Ti is added in combination with V, nitrogen in the molten steel will first react with Ti to form a TiN precipitate, such that a V precipitate cannot be effectively formed, whereby the effect of improving the delayed-fracture resistance of the steel by the V precipitate cannot be obtained. Also, if V is added in combination with Nb, the austenite grains can be refined, but the price of the steel will be inevitably increased, and Nb will interfere with the formation of a V precipitate, because it is highly reactive with nitrogen.
- In addition, the steel wire rod of the present invention comprises Fe and inevitable impurities.
- Hereinafter, the microstructure of the steel wire rod of the present invention will be described in detail.
In the steel wire rod of the present invention, V(C,N) precipitates are preferably distributed in the ferrite structure of pearlite. The V(C,N) precipitates prevent the precipitation of film-like cementite and are distributed in the ferrite structure of pearlite to act as strong hydrogen-trap site, thereby improving the hydrogen delayed-fracture resistance of the steel. - The average particle size of the V(C,N) precipitates is preferably 30 nm or less, and the number of the V(C,N) precipitates is accoding to the invention 1x109/mm2 or more.
- If the size of the V(C,N) precipitates is more than 30 nm, these precipitates will not be finely distributed in the ferrite matrix of pearlite, and thus the effect of increasing the strength of the steel through the uniform distribution of the precipitates will not be obtained. On the other hand, if the V(C,N) precipitates are coarse, they can form coarse precipitates in ferrite to cause fractures, rather than improving the tensile strength of the steel by suppressing the movement of dislocations. For these reasons, the size of the precipitates is preferably 30nm or less.
- Further, the reason for which the number of the precipitates must be 1x109/mm2 or more is that, if the number of the precipitates is less than 1x109/mm2, it will be difficult to ensure a precipitation hardening effect by the V precipitate, and thus the strength sought in the present invention cannot be achieved. If the number of the precipitates is too large, the precipitation hardening effect can be maximized to cause wire breakage during wire drawing.
- Also, the steel wire rod of the present invention has a pearlite structure. As the lamellar spacing of the pearlite structure decreases, the tensile strength and ductility of the wire rod increase. The lamellar spacing of the pearlite structure of the steel wire rod according to the present invention is 150∼300 nm.
- The ductility and strength of pearlite depend on the lamellar spacing of the pearlite. Particularly, the yield strength of pearlite depends on the lamellar spacing thereof, and this can be expressed by the Hall-Petch relationship. Thus, the lamellar spacing needs to be maintained at a suitable level, because a decrease in the lamellar spacing leads to an increase in strength and ductility.
- If the lamellar spacing is less than 150 nm, the strain hardening rate of the wire rod will be excessively increased to cause wire breakage during wire drawing. On the other hand, if the lamellar spacing is more than 300 nm, shear failures, such as cleavage fractures, will be highly likely to occur, and it will be difficult to ensure the strength described below.
- Also, the content of diffusible hydrogen in the wire rod of the present invention is limited to 0.6∼0.9 ppm. The term content of diffusible hydrogen refers to the highest concentration at which steel can contain hydrogen. The content of diffusible hydrogen varies depending on a matrix structure. If the content of diffusible hydrogen in the steel of the present invention is less than 0.6 ppm, the effect of improving resistance to delayed fracture by hydrogen trapping cannot be obtained. The reason for which the content of diffusible hydrogen in the steel wire rod of the present invention is limited to 0.9 ppm is that it is not easy to ensure a diffusible hydrogen content of more than 9 ppm in pearlite steels containing V precipitates, as in the case of the present invention.
- Hereinafter, the method for manufacturing the steel wire rod of the present invention will be described in detail.
- First, a steel satisfying the above-described composition is heated before rolling. Herein, the heating temperature is 1100°C or lower, and preferably 1000∼1100°C.
- The heated steel is subjected to hot rolling. Herein, a process ranging from rough rolling to finish rolling is carried out at a temperature of 900-1000°C.
- The rolled steel is cooled to 650∼600°C at a rate of 5∼10°C/s. If the cooling rate is less than 5°C, proeutectoid cementite will be precipitated to cause anisotropy, and if the cooling rate is more than 10°C/s, martensite, a low-temperature structure, will be formed. The steel cooled after hot rolling has a tensile strength of 1100∼1300 MPa. After the cooling process, the steel is subjected to cold drawing.
- The cold drawing is carried out at a reduction ratio of 60∼80%. In order to ensure the tensile strength of the steel by work hardening through the cold drawing process, the steel is cold drawn at a reduction ratio of 60% or higher. If the reduction ratio is higher than 80%, the cold forgeability of the steel will be deteriorated. For this reason, the upper limit of the reduction ratio is preferably 80%. The cold-drawn wire rod has a tensile strength of 1550∼1650 MPa.
- Hereinafter, the present invention will be described in detail with reference to examples, but the scope of the present invention is not limited to these examples.
- Each of steels satisfying the compositions shown in Table 1 below was heated at 1100°C, after which a strain of 0.6 was applied thereto at a strain rate of 10/s at 950°C. Then, the steels were cooled at a rate of 7.5°C/s and drawn to 10∼90%, thereby manufacturing wire rods. In Table 1 below, the inventive examples are steels to which V has been added within the content range specified in the present invention, the conventional example is a steel to which Cr had been added. Meanwhile, comparative examples 1 and 2 are steels that are out of the V content of the present invention, and comparative examples 3 and 4 are steels to which Al had been added in place of V.
- The wire rods manufactured as described above were measured for tensile strength, elongation and surface roughness, and the results of the measurement are shown in Table 2 below. In addition, the manufactured wire rods were measured for microstructure, fracture stress according to diffusible hydrogen content, and the change in tensile strength according to the amount of drawing, and the results of the measurement are shown in
FIGS. 1 to 6 .[Table 1] C Si Mn Cr Al V Conventional example 0.82 0.25 0.8 0.2 - - Inventive example 1 0.82 0.25 0.8 - - 0.05 Inventive example 2 0.82 0.25 0.8 - - 0.1 Comparative example 1 0.82 0.25 0.8 - - 0.15 Comparative example 2 0.82 0.25 0.8 - - 0.2 Comparative example 3 0.82 0.25 0.8 - 0.04 - Comparative example 4 0.82 0.25 0.8 - 0.08 - [Table 2] Tensile strength (TS, MPa) Elongation (El, %) Surface roughness (RA, %) Conventional example 1051 7 - Inventive example 1 1062.8 7.5 16.7 Inventive example 2 1106.8 6.7 12.2 Comparative example 1 1070.3 7.3 15.2 Comparative example 2 1146.5 5.1 14 Comparative example 3 990.9 6.8 14.5 Comparative example 4 1027.8 7.4 15.1 - As can be seen in Table 2 above, inventive examples 1 and 2 could achieve excellent strength, elongation and surface roughness (RA), which were comparable to the conventional example and comparative examples 3 and 4, even when a trace amount of V was added thereto. Also, as can be seen in comparative examples 1 and 2, even when V was added in an amount higher than the upper limit of the content range specified in the present invention, strength and elongation were not improved. For this reason, the content of V was limited to 0.05∼0.1% in view of diffusible hydrogen content and fracture stress.
- Meanwhile,
FIGS. 1A, 1B ,2A and 2B are photographs showing the results of observing the microstructures of the conventional example and inventive example 1, respectively. As shown inFIGS. 1 and2 , the lamellar spacing of pearlite in inventive example 1 (about 184.3 nm) was about 50% smaller than the conventional example (about 368.75 nm). -
FIGS. 3A through 3F show the results of observing the microstructures of the conventional example, inventive example 1, inventive example 2, comparative example 1, comparative example 2 and comparative example 4, respectively. As shown inFIG. 3 , inventive examples 1 and 2 had lamellar spacing gaps of 184.3 nm and 213 nm, respectively, which were smaller than the conventional example and the comparative examples. -
FIGS. 4A and 4B schematically show the microstructures of the conventional example and inventive example 1, respectively. As can be seen therein, in the conventional example, film-like cementite was precipitated, but in inventive example 1, spherical V(C,N) precipitates were distributed. -
FIG. 5 shows the results of comparing hydrogen delayed-fracture resistance between inventive example 1 and the conventional example (82BC steel). As can be seen therein, at a fracture stress of about 1500 MPa, inventive example 1 had a diffusible hydrogen content of 0.87 ppm which was about 1.5 times higher than the conventional example (diffusible hydrogen content: 0.52 ppm). This is because the spherical V(C,N) precipitates shown inFIG. 4 were segregated in the ferrite-cementite of prior pearlite, whereby diffusible hydrogen was trapped by the spherical V(C,N) precipitates, thus improving the hydrogen delayed-fracture resistance of the wire rod. -
FIG. 6 is a graphic diagram showing a change in tensile strength according to a decrease in diameter in various drawing amounts. As can be seen in Table 2 above andFIG. 6 , the case in which V was added showed excellent tensile strength. If V is added in an amount of more than 0.1%, the increase in tensile strength caused by the formation of V precipitates can be expected, but in comparative examples 1 and 2 in which V was added in large amounts, the wire rods could be cracked or fractured due to the formation of excessive V precipitates. - In comparative examples 3 and 4 in which Al was added, the colony size of the wire rods was advantageously decreased due to grain refinement, Al precipitates were not easily formed due to high cooling rate, and thus it was not easy to correct the strength of the wire rods.
Claims (2)
- An ultra-high-strength steel wire rod having excellent resistance to delayed fracture, the wire rod comprising, by wt%, 0.7-1.2% C, 0.25-0.5% Si, 0.5-0.8% Mn, 0.02-0.1% V, 0.02% or less P, 0.02% or less S, optionally 60ppm or less N and a balance of Fe and inevitable impurities, wherein the wire rod comprises V(C,N) precipitates and the average particle size of the V(C,N) precipitates is 30 nm or less, and the number of the V(C,N) precipitates is 1x109/mm2 or more and the wire rod has a pearlite structure having a lamellar spacing of 150∼300nm, in which the V(C,N) precipitates are distributed in the ferrite structure of the pearlite, and wherein the wire rod has diffusible hydrogen content 0.6∼0.9 ppm.
- A method for manufacturing an ultra-high-strength steel wire rod as per claim 1 having excellent resistance to delayed fracture, the method comprising:heating a steel, which includes, by wt%, 0.7-1.2% C, 0.25-0.5% Si, 0.5-0.8% Mn, 0.02-0.1% V, 0.02% or less P, 0.02% or less S, optionally 60ppm or less N and a balance of Fe and inevitable impurities, to 1100 °C or lower, and hot-rolling the heated steel at a temperature of 900∼1000°C;cooling the rolled steel to 600∼650°C to a rate of 5∼10°C /s; andcold-drawing the cooled steel at a reduction ratio of 60∼80%.
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KR1020090131738A KR20110075319A (en) | 2009-12-28 | 2009-12-28 | Ultra high strength steel wire rod having high resistance of delayed fracture, and method for manufacturing the same |
PCT/KR2010/009281 WO2011081360A2 (en) | 2009-12-28 | 2010-12-23 | Ultra-high-strength steel wire having excellent resistance to delayed fracture and manufacturing method thereof |
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JP6249846B2 (en) * | 2013-03-25 | 2017-12-20 | 株式会社神戸製鋼所 | Steel wire rod for high strength spring excellent in wire drawing workability and bending workability after wire drawing work, method for producing the same, high strength spring, and method for producing the same |
JP6182489B2 (en) * | 2014-03-27 | 2017-08-16 | 株式会社神戸製鋼所 | Case-hardened steel that has excellent cold forgeability and can suppress abnormal grain generation during carburizing. |
JP2016014169A (en) * | 2014-07-01 | 2016-01-28 | 株式会社神戸製鋼所 | Wire rod for steel wire and steel wire |
KR101676130B1 (en) * | 2014-12-19 | 2016-11-15 | 주식회사 포스코 | Wire rod having high strength and ductility and method for manufacturing the same |
JP6461672B2 (en) * | 2015-03-27 | 2019-01-30 | 株式会社神戸製鋼所 | Bolt steel wire and bolt with excellent cold forgeability and delayed fracture resistance after quenching and tempering |
JP6600996B2 (en) * | 2015-06-02 | 2019-11-06 | 日本製鉄株式会社 | High carbon steel sheet and method for producing the same |
JP2017101296A (en) * | 2015-12-02 | 2017-06-08 | 株式会社神戸製鋼所 | Hot rolled wire excellent in hydrogen blistering resistance |
JP2018162524A (en) * | 2018-06-22 | 2018-10-18 | 株式会社神戸製鋼所 | Wire material for steel wire, and steel wire |
JP7352069B2 (en) * | 2019-07-26 | 2023-09-28 | 日本製鉄株式会社 | wire rod and steel wire |
WO2022050500A1 (en) | 2020-09-01 | 2022-03-10 | 현대제철 주식회사 | Material for hot stamping, and method for manufacturing same |
BR112023003717A2 (en) * | 2020-09-01 | 2023-03-28 | Hyundai Steel Co | HOT STAMPING MATERIAL AND METHOD OF MANUFACTURING A HOT STAMPING MATERIAL |
WO2022050501A1 (en) | 2020-09-01 | 2022-03-10 | 현대제철 주식회사 | Material for hot stamping and method for manufacturing same |
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JPH11315348A (en) * | 1998-04-30 | 1999-11-16 | Kobe Steel Ltd | High strength wire rod excellent in delayed fracture resistance, its production, and high strength bolt |
JP3940270B2 (en) * | 2000-04-07 | 2007-07-04 | 本田技研工業株式会社 | Method for producing high-strength bolts with excellent delayed fracture resistance and relaxation resistance |
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JP4009218B2 (en) * | 2003-04-07 | 2007-11-14 | 新日本製鐵株式会社 | Bolt with excellent hydrogen embrittlement resistance and method for producing the same |
US8105698B2 (en) * | 2007-01-31 | 2012-01-31 | Nippon Steel Corporation | Plated steel wire for parallel wire strand (PWS) with excellent twist properties |
JP5000367B2 (en) * | 2007-04-13 | 2012-08-15 | 新日本製鐵株式会社 | High strength galvanized bolt with excellent hydrogen embrittlement resistance |
FR2960556B3 (en) * | 2010-05-31 | 2012-05-11 | Arcelormittal Wire France | HIGH-STRENGTH STEEL-SHAPED WIRE FOR MECHANICAL RESISTANT TO HYDROGEN FRAGILIZATION |
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