CN108070785B - High carbon wire rod with excellent ductility and method for producing same - Google Patents

High carbon wire rod with excellent ductility and method for producing same Download PDF

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CN108070785B
CN108070785B CN201711081233.9A CN201711081233A CN108070785B CN 108070785 B CN108070785 B CN 108070785B CN 201711081233 A CN201711081233 A CN 201711081233A CN 108070785 B CN108070785 B CN 108070785B
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wire rod
wire
ductility
cooling
carbon
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CN108070785A (en
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朴焌学
崔锡欢
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Posco Holdings Inc
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    • 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
    • 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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/16—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling wire rods, bars, merchant bars, rounds wire or material of like small cross-section
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
    • B21B3/02—Rolling special iron alloys, e.g. stainless steel
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B37/00—Control devices or methods specially adapted for metal-rolling mills or the work produced thereby
    • B21B37/74—Temperature control, e.g. by cooling or heating the rolls or the product
    • 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
    • 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
    • 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/009—Pearlite

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Manufacturing & Machinery (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)
  • Heat Treatment Of Strip Materials And Filament Materials (AREA)

Abstract

The invention provides a high-carbon wire rod with excellent ductility and excellent ductility even under high alloy conditions and a manufacturing method thereof.

Description

High carbon wire rod with excellent ductility and method for producing same
Technical Field
The present invention relates to a high-carbon wire rod having excellent ductility and a method for manufacturing the same.
Background
High-strength steel wires having a wire diameter of 10mm or less, which are manufactured by heat-treating and processing high-strength wire rods having a wire diameter of 10mm to 20mm, are used throughout the entire industry in various forms to support loads.
Typical forms include a wire rope used for bridges such as suspension bridges and cable-stayed bridges, a PC steel wire used for reinforcing concrete such as concrete piers, a wire rope used for large buildings or structures, and an Anchor rope (Anchor rope) used for supporting offshore oil fields or various structures.
In addition, when a high-strength wire rod is processed into a steel wire by a drawing (drawing) process having an excellent work hardening effect, it is very advantageous to ensure drawing workability if the ductility (ductility) of the material is excellent.
Before the drawing process, in order to improve the strength and work hardening rate at the initial stage of drawing, usually, high-temperature austenitization is followed by lead bath constant temperature heat treatment (LP heat treatment), but before such heat treatment, if the ductility of the material is poor, it is difficult to ensure the ductility after the heat treatment.
Therefore, the ductility of the starting material is important, and particularly, for a material in which heat treatment such as Direct-Line-Patenting (DLP) is omitted, the ductility of the starting material is more important.
Recently, the contents of alloying elements such as C, Si, Mn, and Cr tend to be increased for increasing the strength of steel wire, but the center segregation caused by these alloying elements is increased, and the formation of pro-eutectoid cementite, which hinders ductility, is promoted.
In addition, when the material itself is strengthened, embrittlement sensitivity tends to increase due to a trace amount of hydrogen present in the wire rod, residual stress generated during cooling, and the like, which also becomes a factor of deteriorating ductility of the starting material.
Therefore, in order to obtain a high-strength wire rod excellent in ductility, it is necessary to suppress the production of proeutectoid cementite under high alloy conditions such as C, Si, Mn, and Cr, to form a fine structure advantageous for ensuring ductility, and to minimize the amount of residual hydrogen, residual stress, and the like in the material.
Disclosure of Invention
Technical problem
The present invention aims to provide a high carbon wire rod having excellent ductility even under high alloy conditions and a method for manufacturing the same.
Technical scheme
An aspect of the present invention provides a high carbon wire rod excellent in ductility, comprising carbon (C): 0.9 to 1.1%, silicon (Si): 0.6% -1.5%, manganese (Mn): 0.2% -0.8%, chromium (Cr): 0.2 to 0.8%, aluminum (Al): 0.08% or less, and the balance Fe and other unavoidable impurities, wherein the size of a fracture head (open area) at the time of tensile fracture is 1.4mm or more, and the ductile fracture fraction of the fracture head (open area) is 35% or more.
Another aspect of the present invention provides a method for manufacturing a high-carbon wire rod having excellent ductility, including the steps of: manufacturing a wire rod satisfying the alloy composition; coiling the wire at 580-750 ℃; pre-cooling the coiled wire; after the temperature of the pre-cooled wire surface rises, the wire surface is cooled to 500 ℃ at a cooling speed of 3-10 ℃/s for one time; and after the primary cooling, performing secondary cooling to 400-500 ℃ at a cooling rate of 1.2 ℃/s or less.
Effects of the invention
According to the present invention, a wire rod excellent in ductility with a reduction of area of 15% or more and excellent in hydrogen embrittlement sensitivity with a residual hydrogen content of 1.2ppm or less can be provided.
Drawings
Fig. 1 is a photograph of the breaking head after tensile breaking of the inventive material 1 and the comparative material 1 observed with a stereo optical microscope in one example of the present invention.
Fig. 2 is a photograph of a broken head portion of the inventive material 2 and the comparative material 2 observed with a scanning electron microscope in one example of the present invention.
Detailed Description
In order to improve the strength of the wire rod, the present inventors have made extensive studies on the fact that the wire rod contains a certain amount or more of C, Si, Mn, Cr, and the like as alloy components and can secure excellent ductility. As a result, they have found that a wire rod excellent in ductility with a reduction of area of 15% or more can be provided by optimizing the alloy composition and the production conditions to secure a fine structure advantageous for ensuring ductility, and have completed the present invention.
The present invention is described in detail below.
The high-carbon wire rod excellent in ductility according to one aspect of the present invention preferably contains carbon (C): 0.9 to 1.1%, silicon (Si): 0.6% -1.5%, manganese (Mn): 0.2% -0.8%, chromium (Cr): 0.2 to 0.8%, aluminum (Al): less than 0.08%.
The reason why the alloy composition of the steel wire provided by the present invention is controlled as described above is described in detail as follows. Unless otherwise specified, the content of each element represents wt%.
C：0.9％～1.1％
Carbon (C) is an element that forms cementite in the steel wire, which forms pearlite of a lamellar structure together with ferrite. The pearlite has high strength compared to ferrite, and thus the strength of the material increases as the fraction of cementite is higher. Further, the more uniform and fine the pitch of the layered structure, the more the strength of the material can be improved.
When the content of C is increased, the fraction of cementite increases and the lamella spacing becomes minute, thus being very effective in improving the strength of the wire rod. For this reason, in the present invention, the content of C is preferably 0.9% or more. However, if the content of C is more than 1.1%, the fraction of proeutectoid cementite becomes too large, which causes a problem of reduction in ductility.
Therefore, in the present invention, the content of C is preferably controlled to 0.9% to 1.1%.
Si：0.6％～1.5％
Silicon (Si) is an element that is solid-dissolved in the ferrite of the base structure to strengthen the steel, and is an element necessary for ensuring the final strength when the wire rod is processed into a steel wire. Because the Si forms a silicon-rich layer on the ferrite/cementite interface in pearlite, it functions to suppress the collapse (decomposition) of cementite at the time of drawing.
In order to sufficiently obtain the above-described effects, the content of Si is preferably 0.6% or more. If the Si content exceeds 1.5%, hardenability is greatly increased, and martensite may be generated when a continuous cooling process is performed after wire rod rolling.
Therefore, in the present invention, the content of Si is preferably controlled to 0.6% to 1.5%.
Mn：0.2％～0.8％
Manganese (Mn) is an element that retards pearlite transformation, and has an effect of easily generating fine pearlite even at a slightly slow cooling rate.
For the above-mentioned effects, the Mn content is preferably 0.2% or more. However, if the content of Mn is more than 0.8%, hardenability is greatly increased, and martensite may be generated in the continuous cooling process.
Therefore, in the present invention, the Mn content is preferably controlled to 0.2% to 0.8%.
Cr：0.2％～0.8％
Chromium (Cr) makes the lamellar spacing of pearlite fine, and Cr has the effect of suppressing the splitting (fracture) of cementite at the time of drawing to minimize the decrease in strength, like the above-mentioned Si.
In order to sufficiently obtain the above-mentioned effects, it is necessary to add 0.2% or more of Cr, but if the content of Cr is more than 0.8%, hardenability is greatly increased, and martensite may be generated during continuous cooling. Further, chromium is a high-priced element, and has a problem of increasing the production cost.
Therefore, in the present invention, the content of Cr is preferably controlled to 0.2% to 0.8%.
As described above, in order to secure the strength of the starting material and the work hardening rate during drawing, in the present invention, it is preferable to add C, Si, Mn, and Cr in amounts of at least a certain amount, wherein Si and Cr have an effect of suppressing the splitting of cementite, and it is preferable to add 0.6% or more and 0.2% or more, respectively.
Al: below 0.08% (except 0%)
Aluminum (Al) is easily reacted with oxygen and is a typical element for a deoxidation reaction for steel making. However, when Al is present in the steel, it is preferable to control so as not to remain in the steel as much as possible, because the formation of inclusions may be promoted. In addition, Al participates in a diffusion reaction of carbon at a high temperature, thereby suppressing a reaction in which C dissolves from cementite to ferrite at the time of austenitizing heating and maintaining at a high temperature, and possibly promoting a phenomenon in which undissolved cementite remains.
In view of this, in the present invention, the Al is preferably added in an amount of 0.08% or less, except for 0%.
The balance of the present invention is iron (Fe). However, impurities from the raw materials or the surrounding environment may be inadvertently and unavoidably mixed in the conventional manufacturing process, and thus it cannot be excluded that these impurities are mixed in. Any person skilled in the art will know these impurities and therefore they will not be described in detail in this specification.
The fine structure of the high carbon wire material of the present invention satisfying the above alloy composition is a pearlite phase as a base structure. In this case, it is preferable that the austenite grain size in the central portion is 15 μm to 45 μm and the pearlite colony (Nodule) size is 5 μm to 15 μm.
In the present invention, the central portion refers to 1/2D (region) of D when the wire diameter is D (diameter).
Generally, the small size of the pearlite mass in the wire is not favorable for crack propagation, and thus is favorable for improving the ductility of the wire.
The size of this pearlite colony is related to the austenite grain size, and when the prior austenite grain size is small, the pearlite colony is also small.
However, when the size of austenite is too small in hypereutectoid steel (hypereutectoid steel), the formation of proeutectoid cementite is promoted at the grain boundary, and a phenomenon of hindering ductility occurs.
Therefore, in the present invention, it is preferable to control the austenite grain size and the pearlite colony size to 15 μm or more and 5 μm or more, respectively. If the austenite grain size and the pearlite colony size are smaller than 15 μm and smaller than 5 μm, respectively, the formation of pro-eutectoid cementite is promoted, and it is difficult to secure a reduction of area of 15% or more. On the contrary, if the austenite grain size and the pearlite colony size are larger than 45 μm and larger than 15 μm, respectively, the crack growth suppression effect by grain size refinement is very small, and ductility with a reduction of area of 15% or more cannot be secured.
As described above, the wire rod of the present invention in which the austenite grain size and the pearlite colony size are controlled can ensure that the size of the fracture head (open region) at the time of tensile fracture is 1.4mm or more and the ductile fracture fraction of the fracture head (open region) is 35% or more.
The fracture head is a region where fracture occurs first in stretching, and the pore aggregate ductile failure fraction due to stress concentration is relatively higher than other regions. That is, a larger size of the fracture head indicates a higher ductility of the material.
In the present invention, when the size of the fracture head is 1.4mm or more, the ductile fracture fraction is 35% or more over the entire fracture surface, and a wire rod excellent in ductility can be provided.
If the ductile fracture fraction of the fracture head is less than 35%, even if the size of the fracture head is 1.4mm or more, the contribution degree to the ductility of the material is slight, and the reduction of area of the wire rod may be less than 15%.
The wire rod of the present invention has a tensile strength of 1200MPa to 1600MPa, and ensures a residual hydrogen content of 1.2ppm or less, thereby ensuring a desired high strength and high ductility.
When the wire rod is processed into a steel wire, it is preferable to maintain the strength of the wire rod, which is a raw material, at a certain level in order to ensure sufficient strength.
Therefore, in the present invention, it is preferable to secure a tensile strength of 1200MPa or more, but if it exceeds 1600MPa, there is a problem that it is difficult to secure a reduction of area of 15% or more because of excessively high strength.
In addition, hydrogen present in steel is a potential factor for inducing brittleness, and it is preferable to reduce the hydrogen content in steel as much as possible.
In the present invention, the hydrogen content in the wire rod is controlled to 1.2ppm or less, so that the sensitivity to hydrogen embrittlement is low, a sound structure can be secured, and excellent ductility can be secured. If the hydrogen content is more than 1.2ppm, hydrogen embrittlement occurs even if the structure is sound, and there is a problem that it is difficult to secure a reduction of area of 15% or more.
A method of manufacturing a wire rod excellent in ductility according to another aspect of the present invention is described in detail below.
The wire rod of the present invention can be produced as follows: after a wire rod satisfying the alloy composition is manufactured through a series of processes, it is coiled under a controlled temperature condition, and then is subjected to a process of cooling by stages.
The wire may be manufactured by various wire manufacturing techniques known in the art, but is preferably subjected to a series of processes as described below.
First, after a billet satisfying the alloy composition is produced, it is preferably subjected to a heating process for homogenizing it.
By the heating process, the microstructure of the ingot is preferably changed to an austenite single phase.
For this purpose, the heating is preferably performed at a temperature ranging from 950 ℃ to 1100 ℃. If the heating temperature is lower than 950 ℃, it is difficult to secure a temperature range in the subsequent wire rod rolling, and if the heating temperature is higher than 1100 ℃, scale (scale) is generated and the decarburization phenomenon is accelerated, thereby deteriorating the surface quality.
Preferably, the billet heated according to the process is wire-rolled and then cooled to produce a wire rod.
In this case, the wire rod is preferably finish-rolled at a temperature ranging from 900 to 1000 ℃. If the finish rolling temperature is less than 900 ℃, roll breakage may occur due to rolling load, and if the finish rolling temperature is more than 1000 ℃, austenite grains become coarse, which causes a problem that it is difficult to secure desired strength.
After the wire rod is rolled, it is preferably cooled to a temperature range of a subsequent coiling process by a conventional water cooling method.
The wire rod thus produced is preferably subjected to winding and cooling processes to finally produce a wire rod having a desired fine structure and physical properties.
In the present invention, the winding process is preferably performed at a temperature ranging from 580 ℃ to 750 ℃, wherein the winding temperature is based on the temperature of the surface of the wire rod.
When the wire rod is coiled after being rolled, the temperature range thereof is an important factor for controlling the size of austenite. It is known that the austenite size of a wire rod generally remains in a nucleated state after rolling, and grows during cooling after coiling. Therefore, if the coiling temperature is high, the temperature range in which austenite can grow is relatively high, and therefore austenite also becomes coarse.
In the present invention, the austenite grain size in the central portion of the wire rod is controlled to be 15 μm to 45 μm, and therefore, it is preferable to perform coiling at a temperature in the range of 580 ℃ to 750 ℃, and more preferably, at a temperature in the range of 590 ℃ to 730 ℃.
If the coiling temperature is lower than 580 ℃, the austenite grain size becomes too small in the central portion of the wire, which promotes the formation of pro-eutectoid cementite, and also causes a low temperature structure in a partially decarburized region, thereby causing a problem that it is difficult to secure ductility with a reduction of area of 15% or more. On the contrary, if the coiling temperature is higher than 750 ℃, the austenite refining effect is insignificant in the central portion of the wire.
After the completion of the coiling process, cooling is preferably performed in stages.
The process of cooling the wire rod is to obtain uniform and fine pearlite, and generally, the surface (surface portion) of the wire rod is accompanied by a phenomenon in which the surface temperature increases after cooling due to a heat return from the inside of the wire rod and a phenomenon of phase transition heat generation. That is, in the present invention, when the wire rod wound as described above is pre-cooled, the temperature increase phenomenon occurs on the surface of the wire rod as described above. In this case, the pre-cooling may be carried out by a conventional cooling process, and the conditions thereof are not particularly limited, and for example, air cooling, spray cooling, or the like may be employed.
For this reason, the present inventors have technically characterized that the cooling process after the temperature increase phenomenon occurs on the wire rod surface as described above is controlled in the cooling rate and the cooling completion temperature to ensure fine pearlite.
Specifically, after the temperature of the surface of the wire rod wound as described above is increased, more preferably, after the temperature of the surface of the wire rod is increased to a temperature range of 600 to 820 ℃, the surface of the wire rod is cooled at a cooling rate of 3 to 10 ℃/s so that the temperature of the surface of the wire rod is decreased from the temperature range to 500 ℃. More preferably, the cooling is performed at a cooling rate of 3.5 ℃/s to 9.5 ℃/s. The cooling at this time is defined as the primary cooling step of the present invention.
When the cooling rate of the primary cooling step is less than 3 ℃/s, since the cooling rate of the central portion of the wire rod is not fast enough, fine pearlite may not be ensured, and the ensured strength may be less than 1200 MPa. In contrast, when the cooling rate is more than 10 ℃/s, uniform pearlite such as pearlite generated in a shape of a split or the like cannot be obtained since the cooling rate of the central portion of the wire rod becomes excessively fast. Further, it is also disadvantageous in securing ductility, and the reduction of area is likely to be less than 15%.
After the primary cooling step is completed, it is preferable to cool the steel sheet while controlling the cooling rate in the temperature range of 400 to 500 ℃ to 1.2 ℃/s or less (excluding 0 ℃/s). The cooling at this time is defined as the secondary cooling step of the present invention, and the temperature range is based on the surface temperature of the wire rod.
In the invention, the secondary cooling is to reduce the content of residual hydrogen in the wire rod, and after the primary cooling is finished, the cooling speed is controlled to be slow during the secondary cooling so as to prolong the time of staying at high temperature, so that the hydrogen in the wire rod is easy to diffuse, and finally the content of residual hydrogen in the wire rod can be reduced.
If the cooling rate is higher than 1.2 ℃/s, the residual hydrogen content in the wire rod is higher than 1.2ppm, and the reduction of area cannot be ensured to be 15% or more.
The present invention is described more specifically by examples below. However, the following examples are only for describing the present invention in more detail and are not intended to limit the scope of the present invention. The scope of rights of the present invention depends on the content reasonably derived from the content recited in the claims.
(examples)
After casting an ingot (70Kg) satisfying the alloy composition shown in the following Table 1, the ingot was heated in a heating furnace at 1100 ℃ for about 2 hours, and then taken out of the heating furnace and subjected to wire rolling at a temperature of 900 ℃ or higher to a wire diameter of 13 mm. Then, the rolled wire rod was cooled to the winding temperature shown in table 1 by water cooling. Thereafter, the wound wire was stood on a stelmor line, and then the surface temperature of the wire was controlled by adjusting the amount of air blown, and the wire was once cooled to 500 ℃ at a cooling rate shown in the following table 1, and then secondary cooling was performed. In this case, the secondary cooling rate in the temperature range of 400 to 500 ℃ is controlled by adjusting the air blowing amount and the heat insulating cover. In this case, the primary cooling and the secondary cooling are performed after the surface temperature of the wire rod wound while standing on the stelmor line is raised to 620 to 800 ℃ by phase transition heat generation.
After confirming the fine structure of each wire rod having been subjected to the stepwise cooling and producing a tensile sample, the tensile strength was measured, and the size of the fracture head and the ductile fracture fraction were measured. In addition, the residual hydrogen content was measured after the wire rod was manufactured.
The tensile test was carried out at room temperature at a speed of 130mm and 20mm/min at a clamp pitch after each wire rod having a wire diameter of 13mm was cut into a sample having a length of 400 mm. The wire diameter of the material measured at the fracture surface portion when the tensile fracture surfaces are butted is DiThe wire diameter before the test is D0When the reduction of area is 100 × (D)i/D0)2。
Then, the fracture head and ductile fracture fraction of the sample after the tensile test were measured using a stereo optical microscope and a scanning electron microscope.
Further, the microstructure of the cross section of the portion 5mm below the fracture surface was observed by a scanning electron microscope, and the crystal grain size was measured. As for the pearlite colony (non) size, after the crystal orientation of the ferrite structure was measured by using an ebsd (electron backscattering diffraction) apparatus for the above cross section, the Boundary of the pearlite colony (non) was defined when the orientation error (misorientation) of the adjacent ferrite crystals was 7 ° or more, and the size of the Boundary (Boundary) drawn at this time was measured.
The amount of hydrogen released when the material from which scale foreign matter was removed was measured by a mass analyzer (Quadrupole mass analyzer) for the amount of residual hydrogen when heated to 800 ℃.
[ TABLE 1 ]
Figure BDA0001458054010000111
Figure BDA0001458054010000121
In table 1, comparative materials 1 to 12 are those in which the alloy composition satisfies the present invention, but the case where the manufacturing process does not meet the present invention is shown as comparative materials.
[ TABLE 2 ]
Figure BDA0001458054010000122
Figure BDA0001458054010000131
In table 2, γ represents austenite, and P represents pearlite.
As shown in the above tables 1 and 2, the alloy composition and the manufacturing conditions both satisfy the inventive materials 1 to 17 of the present invention, the tensile strength thereof satisfies 1200MPa to 1600MPa, and the austenite size and the pearlite colony size as well as the ductile fracture fraction and the reduction of area of the central portion of the wire rod satisfy the ranges proposed in the present invention, so that the wire rod excellent in ductility can be provided. Further, the steel of the wire rod of the present invention has a residual hydrogen content of less than 1.2ppm, and it is confirmed that the wire rod has excellent hydrogen embrittlement sensitivity.
In contrast, comparative materials 1 to 3, which had a temperature higher than 750 ℃ at coiling, had austenite and pearlite colony sizes of more than 45 μm and 15 μm, respectively, and thus the size of the fracture head at tensile fracture was less than 1.4mm, and the ductile fracture fraction of the fracture head was less than 35%, and the reduction of area was less than 15% ductility difference.
In addition, in comparative materials 4 to 6 having a temperature of less than 580 ℃ at the time of coiling, the sizes of austenite and pearlite colonies are too small, and therefore the formation of pro-eutectoid cementite is promoted in the central portion of the wire. Therefore, the size of the fracture head at the time of tensile fracture is less than 1.4mm, the ductile fracture fraction of the fracture head is less than 35%, and the reduction of area is less than 15% ductility difference.
In addition, for comparative materials 7 and 8 having a primary cooling rate of more than 10 ℃/s, the strength of the wire rod is more than 1600MPa, the size of the fracture head at the time of tensile fracture and the ductile fracture fraction exceed the present invention, and therefore the reduction of area is less than 15% ductility difference.
Further, in the comparative materials 9 and 10 in which the primary cooling rate was less than 3 ℃/s, the strength of the wire rod was less than 1200MPa, and the desired high strength could not be secured.
The comparative materials 11 and 12 had a secondary cooling rate of more than 1.2 c/s, and thus the residual hydrogen content in the wire rod was more than 1.2ppm, the size of the fracture head at the time of tensile fracture and the ductile fracture fraction exceeded the present invention, and thus the reduction of area was less than 15% ductility difference.
Fig. 1 is a photograph showing a fracture head after tensile fracture of the invention material 1 and the comparative material 1 observed with a stereo optical microscope, and fig. 2 is a photograph showing a fracture head of the invention material 2 and the comparative material 2 observed with a scanning electron microscope.
As shown in fig. 1, inventive material 1 has a larger fracture head than comparative material 1.
Furthermore, as shown in fig. 2, the fraction of ductile failure of inventive material 2 was higher than that of comparative material 2.

Claims (6)

1. A high-carbon wire rod excellent in ductility, comprising carbon (C): 0.9 to 1.1%, silicon (Si): 0.6% -1.5%, manganese (Mn): 0.2% -0.8%, chromium (Cr): 0.2 to 0.8%, aluminum (Al): less than 0.08% except 0%, and the balance Fe and other inevitable impurities,
the fine structure contains a pearlite phase as a base structure, and the austenite grain size in the central portion is 15 to 45 μm, the size of the pearlite colony is 5 to 15 μm,
the wire has a tensile strength of 1200MPa to 1600MPa and a reduction of area of 15% or more.
2. The high-carbon wire rod excellent in ductility according to claim 1, wherein a size of a fracture head at the time of tensile fracture is 1.4mm or more, and a ductile fracture fraction of the fracture head is 35% or more.
3. The high carbon wire rod excellent in ductility according to claim 1, wherein the residual hydrogen content of the wire rod is 1.2ppm or less.
4. A method for manufacturing a high-carbon wire rod excellent in ductility, comprising the steps of:
manufacturing a wire comprising, in weight percent, carbon (C): 0.9 to 1.1%, silicon (Si): 0.6% -1.5%, manganese (Mn): 0.2% -0.8%, chromium (Cr): 0.2 to 0.8%, aluminum (Al): 0.08% or less except 0%, and the balance being Fe and other inevitable impurities;
coiling the wire at 580-750 ℃;
pre-cooling the coiled wire;
after the temperature of the pre-cooled wire surface rises, the wire surface is cooled to 500 ℃ at a cooling speed of 3-10 ℃/s for one time; and
after the primary cooling, the mixture is cooled to 400-500 ℃ at a cooling rate of 1.2 ℃/s or less.
5. The method of manufacturing a high-carbon wire rod excellent in ductility according to claim 4, wherein,
the primary cooling is performed after the surface of the pre-cooled wire rod is raised to the temperature range of 600-820 ℃.
6. The method of manufacturing a high-carbon wire rod excellent in ductility according to claim 4, wherein,
the step of manufacturing a wire rod comprises the steps of: manufacturing a blank; heating the blank at 950-1100 ℃; performing final wire rod rolling on the heated blank at 900-1000 ℃ to manufacture a wire rod; and water-cooling the wire rod to a coiling temperature.
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