US20120128524A1

US20120128524A1 - Steel wire rod having excellent cold heading quality and hydrogen delayed fracture resistance, method of manufacturing the same, and mehod of manufacturing bolt using the same

Info

Publication number: US20120128524A1
Application number: US13/270,856
Authority: US
Inventors: Young Soo CHUN; Chong Soo Lee; You Hwan LEE; Kyung Tae Park; Tae Kyung Lee; Sung Hyuk Park; Chan Hee Park; Ji Soo Kim; Young Kook Lee
Original assignee: Academy Industry Foundation of POSTECH
Current assignee: Academy Industry Foundation of POSTECH
Priority date: 2010-11-22
Filing date: 2011-10-11
Publication date: 2012-05-24

Abstract

Provided are a high-strength, high-manganese steel wire rod having excellent cold heading quality and not requiring spheroidizing and quenching-tempering treatments during manufacturing a bolt and a method of manufacturing a bolt using the steel wire rod. The method of manufacturing a steel wire rod includes heating a steel containing 12 to 25 wt % of Mn within a temperature range of 1100° C. to 1250° C., hot rolling the heated steel within a temperature range of 700° C. to 1100° C., and cooling the hot rolled steel to a temperature of 200° C. or less and cold caliber rolling or drawing to manufacture a steel wire rod.

Description

CROSS REFERENCE

This application claims foreign priority under Paris Convention and 35 U.S.C. §119 to Korean Patent Application Nos. 10-2010-0116350, filed 22 Nov. 2010, and 10-2011-0022061 filed 11 Mar. 2011, with the Korean Intellectual Property Office.

BACKGROUND OF THE INVENTION

1. Technical Field
The present invention relates to a steel wire rod used for a bolt, and more particularly, to a high-strength, high-manganese steel wire rod, which has excellent cold heading quality and hydrogen delayed fracture resistance and is enabled to skip spheriodizing and quenching-tempering heat treatment processes, a method of manufacturing the same, and a method of manufacturing a bolt using the same.
2. Description of the Related Art
There is a trend towards continuously increased demand for a high-strength wire rod in order to achieve energy efficiency increase and energy consumption decrease. For example, when bolts used in an automobile engine are replaced by high-strength bolts to reduce the weight of bolts by 80 g, a ripple effect on the associated components, i.e., 20 kg of engine weight reduction, will be very large. Therefore, continuous research and development on the application of high-strength steel for a component material have been required.
However, an obstacle in manufacturing of high-strength bolt steel is that cold heading quality deteriorates due to the use of high carbon steel for high strengthening. With respect to the high-strength steel wire rod having poor cold heading quality, life time of cold heading dies is decreased and this will cause an increase in production costs.
As a method for resolving the foregoing limitation, typical tempered martensitic steels are used as a material for a high-strength wire rod. However, with respect to the foregoing martensitic steels, post processing has to be performed, in which cold heading is performed after being subjected to a spheriodizing heat treatment requiring for about 20 hours or more to obtain low flow stress in order to improve cold heading quality, and thereafter, quenching and tempering heat treatments are performed to increase strength.
The spheroidizing heat treatment and the quenching and tempering processes before and after the cold heading cause cost increase and generate distortion during heat treatments with respect to a large bolt. In order to prevent the foregoing, various types of research have been recently conducted for manufacturing a heat treatment free high-strength bolt.
However, with respect to a general heat treatment free bolt developed to date, cold cold heading quality is excellent but high strength has not been achieved. When a high-strength steel wire rod is used, material stability was not guaranteed due to the generation of defects in a bolt head portion and the existence of residual stress, according to cold heading of the high-strength wire rod having poor cold heading quality.
Meanwhile, twinning induced plasticity (TWIP) steel, which is high-manganese steel, relieves strain energy by using twins. The movement of dislocations is easier in comparison to a body centered cubic (BCC) structure because many slip systems exist in the TWIP steel due to the characteristics of its face centered cubic (FCC) crystal structure. Since the deformation of the TWIP steel is accomplished by means of twins and dislocations, cold heading quality of the TWIP steel is very good and a strengthening effect is dynamically induced by using twins generated during deformation. Such nano-sized twins maximize a grain refinement effect and thus, a high-strength material may be obtained.
Korean Patent Application Laid-Open Publication No. 0851158 discloses a related art with respect to the foregoing high-manganese steel. The foregoing patent is related to a high-manganese, high-strength steel plate having excellent impact properties and a method of manufacturing the same, and describes about a method of manufacturing a plate by using high-manganese steel containing 10 wt % to 25 wt % of manganese (Mn). This technique may be applied to a plate, but the technique is inappropriate for manufacturing a wire rod through a caliber rolling mill or drawing which has a different deformation mode.
Also, the biggest obstacle in strengthening of a wired rod is hydrogen delayed fracture that deteriorates material stability. The hydrogen delayed fracture is generated by the reduction of the cohesive strength of a material in such a manner that hydrogen atoms from hydrogen sulfide gas or water existing in an external environment penetrate into the material to weaken metallic bonds of the material. Since the hydrogen solubility of steel is very low at room temperature, the hydrogen atoms penetrated into the material exist by being trapped in dislocations, grain boundaries, and interphase boundaries which are energetically stable. As a result, intergranular fracture is generated due to the intensive weakening of bond strength by means of hydrogen at grain boundaries or the like.
In particular, it has been known that hydrogen delayed fracture resistance significantly decreases in high-strength steel having a strength of 1 GPa or more. The reason for this is that diffusible hydrogen trapping sites are increased according to the increase in the density of dislocation as an essential defect in a material and the increase in grain boundary density due to the grain refinement as the material is subjected to high strengthening. Therefore, a wire rod having excellent hydrogen delayed fracture resistance as well as the development of high-strength steel is required.

SUMMARY OF THE INVENTION

The present invention addresses the above-identified, and other problems associated with conventional methods and apparatuses.
An aspect of the invention provides a steel wire rod promoting high strengthening of the steel wire rod as well as having excellent cold heading quality and being enabled to skip spheriodizing and quenching-tempering heat treatment processes, and a method of manufacturing the same.
Another aspect of the invention provides a method of manufacturing a bolt using the steel wire rod.
Another aspect of the invention provides a steel wire rod promoting high strengthening as well as having excellent hydrogen delayed fracture resistance and a method of manufacturing the same.
According to an embodiment of the invention, there is provided a method of manufacturing a steel wire rod having excellent cold heading quality and hydrogen delayed fracture resistance including: heating a steel containing about 12 wt % to about 25 wt % of manganese (Mn) within a temperature range of about 1100° C. to about 1250° C.; hot rolling the heated steel within a temperature range of about 700° C. to about 1100° C.; and cooling the hot rolled steel to a temperature of 200° C. or less and cold caliber rolling or drawing to manufacture a steel wire rod.
In the method of manufacturing a steel wire rod according to the invention, the steel may include about 12 wt % to about 25 wt % of manganese (Mn), about 0.5 wt % to about 1.0 wt % of carbon (C), about 1.0 wt % to about 2.0 wt % of aluminum (Al), residual iron (Fe), and unavoidable impurities.
Also, in the method of manufacturing a steel wire rod according to the invention, the cold caliber rolling or drawing may be performed at a reduction of area range of about 10% to about 70% and for example, the cold caliber rolling may be performed at a reduction of area range of about 30% to about 90%.
According to another embodiment of the invention, there is provided a steel wire rod having excellent cold heading quality and hydrogen delayed fracture resistance including about 12 wt % to about 25 wt % of manganese (Mn), wherein a microstructure of the steel wire rod includes a face centered cubic austenitic structure and a <112>{111} twin system.
In the method of manufacturing a steel wire rod according to the invention, the steel wire rod includes a total of twelve twin systems in four <112> orientations and three {111} planes of a lattice and four twin variants are formed in one plane.
In the method of manufacturing a steel wire rod according to the invention, a composition of the steel wire rod may include about 12 wt % to about 25 wt % of manganese (Mn), about 0.5 wt % to about 1.0 wt % of carbon (C), about 1.0 wt % to about 2.0 wt % of aluminum (Al), residual iron (Fe), and unavoidable impurities.
According to another embodiment of the invention, there is provided a method of manufacturing a bolt including: cold heading a steel wire rod manufactured by the method of manufacturing a steel wire rod according to the embodiment of the invention to manufacture a bolt head; and performing a heat treatment on the cold headed steel wire rod within a temperature range of about 400° C. to about 600° C.
In the method of manufacturing a bolt according to the invention, the heat treatment may be performed for about 10 minutes or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating a manufacturing history of a steel wire rod according to an embodiment of the invention;

FIGS. 2( a) and 2(b) are a tensile curve and an optical micrograph of initial hot rolled steel, respectively;

FIGS. 3( a) to 3(d) illustrate image maps of back scattered electron diffraction patterns of hot rolled steel and cold caliber rolled steel wire rods having a reduction of area of 31%, 43%, and 54%, respectively;

FIGS. 4( a) to 4(d) are true stress-true strain curves illustrating tension and compression measured from hot rolled steel and cold caliber rolled steel wire rods having a reduction of area of 31%, 54%, and 82%, respectively;

FIGS. 5( a) to 5(c) are tensile curves after heat treating cold caliber rolled steel wire rods having a reduction of area of 31%, 43%, and 54% at 550° C. and 600° C. for 8 minutes, respectively;

FIGS. 6( a) and 6(b) illustrate an image map and a grain map of back scattered electron diffraction patterns of Comparative Example, and 6(c) and 6(d) illustrate an image map and a grain map of back scattered electron diffraction patterns of Inventive Example 3, respectively;

FIGS. 7( a) and 7(b) illustrate transmission electron micrographs of a plate rolled steel sheet and a caliber rolled wire rod, respectively;

FIG. 8 is a graph illustrating changes in strength with respect to deformation according to a reduction of area during caliber rolling; and

FIG. 9 is a graph illustrating notch fracture strength according to a diffusible hydrogen content.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Hereinafter, the invention is described in detail.
The present inventors developed a high-strength steel wire rod having excellent cold heading quality by using a dynamic strengthening deformation mechanism of twinning induced plasticity (TWIP) steel. The TWIP steel has excellent elongation and is enabled to omit heat treatments even though it is high-strength steel. It may be understood that the TWP steel had infinite strain without cracks according to the results of a compression test performed on 1800 MPa class high-strength steel, which had been pre-deformed by indirect multi-axial tension using caliber rolling.
The reason for this is that when compressive stress is exerted on a specimen under applied tensile stress, low yield strength is obtained according to the characteristics of the TWP steel having high back stress known as the Bauschinger effect, and continuous deformation and high strengthening are achieved by means of a twin mechanism.
Also, in a body centered cubic crystal structure, hydrogen diffusion occurs from an octahedral interstial site to a nearest octahedral interstial site and thus, hydrogen delayed fracture occurs. In contrast, in a face centered cubic crystal structure, stable hydrogen at a tetrahedral interstial site diffuses to a nearest tetrahedral interstial site through an octahedral interstial site and thus, hydrogen diffusion in the face centered cubic crystal structure will be about 100 to 1000 times slower than that of the body centered cubic crystal structure. Further, since hydrogen cracking of steel is a very low value of about 10⁻⁴ppm at room temperature, hydrogen penetrated into the steel mainly exists by being trapped at microstructural defects such as dislocations and grain boundaries. A typical steel wire rod secures strength through high dislocation density and fine grains such that it is vulnerable to hydrogen trap. However, if the strength is secured by means of high twin boundary density instead of increasing dislocation density, hydrogen delayed fracture resistance may be improved because the density of diffusible hydrogen trap sites decreases.
The present inventors have developed a steel wire rod having excellent cold heading quality and hydrogen delayed fracture resistance based on the observations, in which excellent cold heading quality may be obtained through the twin mechanism and Bauschinger effect, and hydrogen delayed fracture resistance may be improved when an austenitic single phase having a face centered cubic structure with a low hydrogen diffusion rate and a deformation structure having mainly twins are included.
First, a method of manufacturing a steel wire rod according to the invention is described in detail.
According to the method of manufacturing a steel wire rod of the invention, steel containing 12 wt % to 25 wt % of manganese (Mn) is first heated within a temperature range of 1100° C. to 1250° C. Although a composition of steel except manganese is not particularly limited, the composition of the steel may include 0.5 wt % to 1.0 wt % of carbon (C), 1.0 wt % to 2.0 wt % of aluminum (Al), residual iron (Fe), and unavoidable impurities. However, the addition of other components in addition to the foregoing composition is not excluded.
The heating of the steel within a temperature range of 1100° C. to 1250° C. is for a homogenization treatment of the steel, which is for preventing segregation of elements occurred during casting, and for example, the homogenization treatment may be performed within a temperature range of 1150° C. to 1200° C.
The heated steel is hot rolled within a temperature range of 700° C. to 1100° C. The foregoing hot rolling corresponds to the hot rolling of a wire rod for size adjustment. The hot rolling may be performed within a temperature range of 700° C. to 1100° C. because a decrease in a reduction rate may occur during cold caliber rolling due to the twins generated during the rolling when the hot rolling temperature is less than 700° C. and the decrease in the reduction rate occurs when the temperature is more than 1100° C. due to the fact that grain size becomes so large that twins are not effectively generated during cold caliber rolling.
The hot rolled steel is cooled to a temperature of 200° C. or less and then a wire rod is manufactured by performing cold caliber rolling or drawing. The reason for this is that the reduction of area of a material rather decreases more due to the limitations in that generation of twins is rapid at low strain and the twins are not generated at high strain when the cold caliber rolling temperature is more than 200° C. The reduction of area during the caliber rolling or drawing varies according to the required strength of the wire rod. The reduction of area may be 10% or more in order to increase yield strength and the reduction of area of 90% may be regarded as a possible limit for cold heading. Therefore, the caliber rolling or drawing may be performed within a reduction of area range of 10% to 90% and for example, the reduction of area with respect to cold caliber rolling may be in a range of 30% to 60% according to the use of a wire rod. Also, the lower limit of the temperature is not particularly limited and room temperature may be used.
The strengthening effect of a material same as the grain refinement is obtained during the cold caliber rolling due to the generation of twins in the material. Also, that cold reduction of area and strengthening higher than those of plate rolling may be obtained may be explained through transmission electron micrographs of a plate rolled steel sheet and a caliber rolled wire rod as shown in FIG. 7. As shown in FIG. 7( b), different from FIG. 7( a) in which the plate rolling is performed, all four twin variants are generated during caliber rolling, which is different from the fact that only a single twin variant is generated during typical plate rolling. According to the formation of the four twin variants, a grain refinement effect is maximized such that the achievement of high strength is facilitated, and the twins in four different orientations act as a main deformation mechanism in a material and thus, high reduction of area may be obtained. Therefore, manufacturing of a steel wire rod having various strengths is facilitated.
Also, since the hydrogen cracking of steel has a very low value of about 10⁻⁴ppm at room temperature, hydrogen penetrated into the steel mainly exists by being trapped at microstructural defects such as dislocations and grain boundaries. A typical steel wire rod secures strength through high dislocation density and fine grains such that it is vulnerable to hydrogen trap. However, a high-manganese steel wire rod according to the invention secures strength by low dislocation density and high twin boundary density based on dynamic strengthening effects as well as improving hydrogen delayed fracture resistance due to the existence of low density diffusible hydrogen trap sites.
Hereinafter, a steel wire rod according to the invention is described in detail.
The steel wire rod of the invention includes 12 wt % to 25 wt % of Mn. A composition of the steel wire rod of the invention may include 0.5 wt % to 1.0 wt % of carbon (C), 1.0 wt % to 2.0 wt % of aluminum (Al), residual iron (Fe), and unavoidable impurities.
The steel wire rod of the invention has high strength and high elongation because twins and dislocations act as a deformation mechanism at the same time during deformation. Also, a fine grain strengthening effect is generated by nano-sized twins induced during deformation as well as increasing elongation by relieving strain energy. Low yield strength, which is a limitation of the hot rolled TWIP steel, may be increased through cold caliber rolling or drawing. High dislocation density generated during cold deformation increases yield strength of a material. Further, with respect to the cold deformed steel, cold heading quality for a subsequent process is also excellent according to a deformation mechanism by means of twins.
The steel wire rod of the invention is austenitic steel having a face centered cubic structure and has a <112>{111} twin system. A total of twelve twin systems in four <112> orientations and three {111} planes are included in a lattice and at this time, formation of four twin variants is possible in one plane. The main strengthening mechanism is an interaction between the four twin variants and dislocations. A particular point is that all the dislocations interacting with the twins of the steel wire rod are dislocations in a tensile direction. When the deformation in a direct tensile or an indirect tensile direction is applied to the steel wire rod, dislocations only in a tensile direction are formed and these dislocations contribute to the improvement of cold forgeablity later. That is, flow stress of a material is lowered through offsetting the dislocations in a tensile direction with dislocations in a compression direction during compressive deformation, and thus, cold heading is facilitated.
Also, the high-manganese steel wire rod according to the invention has a single phase austenitic structure. Since the austenitic steel has a face centered cubic (FCC) structure having a low hydrogen diffusion rate, mobility of hydrogen trapped in the austenitic steel is lower than that of typical ferrite steel. Further, the high-manganese steel, in which dynamic strengthening through mechanical twins is achieved instead of strengthening by grain boundaries or ultra-fine carbides acting as diffusible hydrogen trap sites, has a rather positive effect on hydrogen delayed fracture because twins act as the trap sites of non-diffusible hydrogen. The reason for this is that trap activation energy with respect to hydrogen is large because the mechanically formed twins are not previously known commensurate interfaces but are non-commensurate interfaces. Therefore, the high-manganese steel wire rod has very good hydrogen delayed fracture resistance because it may have both of low hydrogen diffusion rate and high hydrogen trap activation energy.
Hereinafter, a method of manufacturing a bolt using a steel wire rod according to the invention is described in detail.
In the manufacturing of the bolt using the steel wire rod according to the invention, cold heading is performed to form a bolt head portion. In a typical case, a spheroidizing heat treatment is performed in order to improve cold heading quality, but cold heading may be directly performed by omitting the spheroidizing heat treatment in the invention. Since elongation is very low with respect to a typical high-strength bolt, a low-strength spheroidized material is cold forged and thereafter, a high-strength quenching/tempering (Q/T) heat treatment has to be performed. However, the shape of the material may be distorted when the foregoing Q/T heat treatment is performed and economy deteriorated because the spheroidizing heat treatment takes about 20 hours.
The cold heading is performed, and then a stress-relief heat treatment is performed within a temperature range of 400° C. to 600° C. The stress-relief heat treatment may be performed within a temperature range of 400° C. to 600° C. in which recrystallization does not occur and the heat treatment time may not be more than 10 minutes. Heating may be performed at a temperature of 400° C. or more because redistribution of dislocations does not occur at a temperature of less than 400° C. The stress-relief heat treatment redistributes tangled dislocations to relieve residual stress, and thus, contributes to increase the stability of a material by removing local stress concentration sites.
The invention has an advantage in comparison to the related art in that separate quenching and tempering are not performed after cold heading is performed.
Hereinafter, characteristics of a steel wire rod manufactured according to an embodiment of the invention are described in detail.
Steel wire rods were manufactured by performing cold caliber rolling with various reductions of area after performing a homogenization treatment by heating at 1200° C. and hot rolling at 1100° C. on a steel including 0.6 wt % C, 18 wt % Mn, 1.5 wt % Al, residual Fe, and unavoidable impurities according to a manufacturing history of FIG. 1.
An initial tensile curve and an optical micrograph of the hot rolled steel were shown in FIGS. 2( a) and 2(b), respectively. FIG. 2( a) shows an initial tensile curve of the hot rolled steel and it may be understood that the hot rolled steel had a yield strength of 309 MPa, a tensile strength of 736 MPa, and an elongation of 60%. The foregoing steel had high strength by means of the interaction between twins-dislocations through the formation of twins and has high elongation because the twins uniformly formed in the steel relieve local stress concentration sites. It may be confirmed in FIG. 2( b) that the microstructure had equiaxed grains with annealing twins formed therein.
Meanwhile, FIG. 3 shows image maps of back scattered electron diffraction patterns from the microstructures of (a) hot rolled steel having an initial grain size of 14.3 μm and cold caliber rolled steel wire rods having a reduction of area of (b) 31%, (c) 43%, and (d) 54%, respectively. From FIGS. 3( a) to 3(d), it may be observed that the structure of mechanical twins changed from a single twin variant to four twin variants having <112> orientations in {111} planes as the reduction of area increases. The mechanical twins induced by deformation act as obstacles reducing a mean free path of dislocations and thus, increase the strength of a material by acting as a strengthening mechanism having a grain refinement effect on the steel wire rod.
Also, in order to investigate true tensile (compression) properties, the true tensile (compression) properties of the hot rolled steel and the cold caliber rolled steel wire rod having a reduction of area of 54% were evaluated. For the foregoing evaluation, tensile tests were performed with round tensile specimens having 3 mm diameters and 12.5 mm gauge lengths, and compression tests were performed by using cylindrical compression specimens having 2.8 mm diameters and 4.2 mm lengths. The results thereof are presented in FIGS. 4( a) and 4(b), respectively. FIG. 4( a) was the result of the hot rolled steel and FIG. 4( b) was the result of the cold caliber rolled steel wire rod having a reduction of area of 54%.
As shown in FIG. 4, both of the hot rolled steel and the cold caliber rolled steel wire rod had excellent compression properties. However, compression cracks were not generated and infinite compression strain was obtained even though the cold caliber rolled steel wire rod having a reduction of area of 54% was an ultra-high strength steel wire rod having a tensile strength of 1830 MPa. Excellent compression properties were shown because tensile dislocations and back stress formed during cold caliber rolling exhibit the Bauschinger effect to obtain low compressive flow stress, and excellent elongation was obtained during the compression test because twins, which can be formed in twelve {111}<112> orientations, were formed only in four orientations during tension and it left room for forming twins in other eight orientations.
Meanwhile, FIG. 5 shows tensile curves after heat treating the cold caliber rolled steel wire rods having various reductions of area at 550° C. and 600° C. for 8 minutes. In FIG. 5, stress-relief heat treatments were applied to provide stabilities to the cold caliber rolled steels having low uniform strain ε_peakwith respect to total strain ε_tot. As shown in FIGS. 5( a) to 5(c), it may be confirmed that uniform strain was increased without generating a decrease in strength when the stress-relief heat treatments were performed.
Manufacturing of bolt products using a typical steel wire rod had limitations of long product shipping time due to spheroidizing and quenching-tempering heat treatments and distortion of the bolt products due to the heat treatments. However, the steel wire rod of the invention has excellent cold heading quality so that a heat treatment may be omitted and thus, may be used as an innovative steel wire rod for next-generation component and material industries.
Next, in order to evaluate hydrogen delayed fracture resistance of the steel wire rod according to the invention, steel wire rods were manufactured by performing cold caliber rolling after a homogenization treatment was performed by heating at 1200° C. and hot rolling was performed at 1100° C. on a steel including 0.6 wt % C, 18 wt % Mn, and 1.5 wt % Al in Fe according to the manufacturing history of FIG. 1.
According to the reduction rates during cold caliber rolling of the steel wire rod, a sample caliber rolled at a reduction of area of 82% (Inventive Example 1), a sample caliber rolled at a reduction of area of 70% (Inventive Example 2), a sample caliber rolled at a reduction of area of 54% (Inventive Example 3), a sample caliber rolled at a reduction of area of 43% (Inventive Example 4), and a sample caliber rolled at a reduction of area of 31% (Inventive Example 5) were prepared, respectively.
Meanwhile, a sample having a tempered martensitic structure was manufactured as a Comparative Example of the invention by hot rolling and oil quenching after a heat treatment was performed at 900° C. for 30 minutes without cold caliber rolling, and then air cooling after a heat treatment was performed at 460° C. for 90 minutes.
Also, a sample having a pearlite structure was manufactured as a Comparative Example by heating at 1000° C. for 10 minutes after hot rolling, and then quenching to 520° C. through lead patenting and air cooling.
FIGS. 6( a) and 6(b) illustrate an image map and a grain map of back scattered electron diffraction patterns of Comparative Example and 6(c) and 6(d) illustrate an image map and a grain map of back scattered electron diffraction patterns of Inventive Example 3, respectively. As shown in FIGS. 6( c) and 6(d), it may be confirmed in the invention that microstructures having very dense mechanical twins were obtained by performing cold caliber rolling. The deformation-induced mechanical twins act as obstacles decreasing a mean free path of dislocations and thus, increase the strength of a material by acting as a strengthening mechanism having a grain refinement effect on the steel wire rod.
FIG. 8 is a graph illustrating changes in strength according to strain in Inventive Examples 1 to 5 and Conventional Example. From the results of FIG. 8, it may be understood that strength and ductility may be changed according to the reduction of area during cold caliber rolling, and the strength may be further increased in comparison to Conventional Example while the ductility is maintained through the cold caliber rolling.
Meanwhile, an anode hydrogen injection method and a low-speed tensile test were performed in order to quantitatively compare hydrogen delayed fracture resistances and the results thereof are shown in FIG. 8. Different hydrogen injected aqueous solutions were used in order to inject the same amount of hydrogen into high manganese austenitic steel of Inventive Example 3, pearlitic steel of Comparative Example, and tempered martensitic steel of Conventional Example.
A hydrogen injection amount of Inventive Example 3 was about 100 times lower than those of Comparative Example and Conventional Example in the same environment because the trap sites of diffusible hydrogen in Inventive Example 3 were smaller than those of Comparative Example and Conventional Example. Therefore, hydrogen injection in Inventive Example 3 was performed for three days at a current density range of 10 A/m²to 50 A/m²in a more harsh aqueous solution, i.e., a mixed solution of ammonium thiocyanate and sodium chloride, and hydrogen injections in Comparative Example and Conventional Example were respectively performed for three days at a current density range of 1 A/m²to 20 A/m²in a less harsh sodium chloride aqueous solution having a concentration of 0.1 normal.
The low-speed tensile test was performed at a strain rate of 10⁻⁵/sec and notch fracture strength was measured by simulating hydrogen diffusion towards a stress center region, and the results thereof are presented in FIG. 9. As shown in FIG. 9, the larger the amount of diffusible hydrogen is, the lower the notch fracture strength is. Degree of embrittlement by hydrogen is also an important point to be noted. In the case of Conventional Example and Comparative Example, the degrees of hydrogen embrittlement (reduction of fracture strength in FIG. 9) were respectively 68% and 58% with respect to about 2 ppm of diffusible hydrogen, but the degree of hydrogen embrittlement was 14% in the case of Inventive Example 3.
According to the invention, a steel wire rod having excellent cold heading quality and being enabled to omit a heat treatment as well as achieving high strength may be provided. Therefore, effects such as manufacturing cost reduction, manufacturing defect reduction, and lightening of components using the steel wire rod may be obtained.
Also, the steel wire rod according to the invention has excellent hydrogen delayed fracture resistance despite ultra-high strength, thereby enabling to contribute to increase the durability and safety of a component.

Claims

1. A method of manufacturing a steel wire rod having excellent cold heading quality and hydrogen delayed fracture resistance, comprising:

heating a steel containing about 12 wt % to about 25 wt % of manganese (Mn) within a temperature range of about 1100° C. to about 1250° C.;

hot rolling the heated steel within a temperature range of about 700° C. to about 1100° C.; and

cooling the hot rolled steel to a temperature of 200° C. or less and cold caliber rolling or drawing to manufacture a steel wire rod.

2. The method according to claim 1,

wherein the steel comprises about 12 wt % to about 25 wt % of manganese (Mn), about 0.5 wt % to about 1.0 wt % of carbon (C), about 1.0 wt % to about 2.0 wt % of aluminum (Al), residual iron (Fe), and unavoidable impurities.

3. The method according to claim 1,

wherein the cold caliber rolling or drawing is performed at a reduction of area range of about 10% to about 70%.

4. The method according to claim 3,

wherein the cold caliber rolling is performed at a reduction of area range of about 30% to about 90%.

5. A steel wire rod having excellent cold heading quality and hydrogen delayed fracture resistance comprising about 12 wt % to about 25 wt % of manganese (Mn),

wherein a microstructure of the steel wire rod comprises a face centered cubic austenitic structure and a <112>{111} twin system.

6. The steel wire rod having excellent cold heading quality and hydrogen delayed fracture resistance according to claim 5,

wherein the steel wire rod comprises a total of twelve twin systems in four <112> orientations and three {111} planes of a lattice and four twin variants are formed in one plane.

7. The steel wire rod having excellent cold heading quality and hydrogen delayed fracture resistance according to claim 5,

wherein a composition of the steel wire rod comprises about 12 wt % to about 25 wt % of manganese (Mn), about 0.5 wt % to about 1.0 wt % of carbon (C), about 1.0 wt % to about 2.0 wt % of aluminum (Al), residual iron (Fe), and unavoidable impurities.

8. A method of manufacturing a bolt, comprising:

hot rolling the heated steel within a temperature range of about 700° C. to about 1100° C.;

cooling the hot rolled steel to a temperature of 200° C. or less and cold caliber rolling or drawing to manufacture a steel wire rod;

cold heading the steel wire rod to manufacture a bolt head; and

performing a heat treatment on the cold forged steel wire rod within a temperature range of about 400° C. to about 600° C.

9. The method according to claim 8,

10. The method according to claim 8,

11. The method according to claim 8,

12. The method according to claim 8,

wherein the heat treatment is performed for about 10 minutes or less.