KR100554752B1 - Method for manufacturing high carbon working product having a earthquake property - Google Patents

Abstract

The present invention relates to a method for manufacturing a steel workpiece such as bolts, the object of which is to heat the graphite before the cold forming (graph) heat treatment to ensure the graphite structure advantageous for cold forming, elongation, impact toughness It is to provide a method for producing a steel workpiece to perform a heat treatment imparting physical properties to secure a microstructure that can improve the overall mechanical properties such as yield ratio.

The present invention for achieving the above object, in weight%, carbon 0.65-1.50%, silicon 2.0-4.0%, manganese 0.1-0.8%, phosphorus 0.01% or less, sulfur 0.01% or less, nitrogen 0.002-0.01%, oxygen 0.002 % Or less, nickel 0.3-2.0%, boron 0.001-0.003%, vanadium: 0.01-0.5%, niobium: 0.01-0.5%, molybdenum 0.01-0.5%, titanium 0.01-0.2%, tungsten 0.01-0.5 %, A heat treatment step of air-cooling by graphitizing the steel composed of one or two or more selected from the group consisting of 0.01-0.2% of copper, remaining Fe and other impurities at a temperature of Ac1- (60 ± 30 ° C),

Cold forming the heat-treated steel into a steel workpiece,

The steel processed product is heated at a temperature of Ac3 transformation point for 20 minutes or more to form austenite single phase, quenched to a temperature of Ms + 220 ° C to Ms + 300 ° C at a cooling rate of 30 ° C / sec or more, and isothermally treated for at least 60 minutes, and Air-cooled, the microstructure has a composite structure of ferrite + residual austenite, and the seismic type high carbon steel processed product comprising the phase fraction of brachy type residual austenite in the residual austenitic is 10 to 30% The technical summary of the method for the production of

Bolt, yield ratio, seismic resistance, wire rod, high carbon

Description

Method for manufacturing high carbon working product having a earthquake property

The present invention relates to a method for manufacturing a steel product such as bolts, and more particularly, to a steel (wire) before cold forming, a graphitization heat treatment for securing a graphite structure advantageous for cold forming, and to an elongated steel product after molding The present invention relates to a method for manufacturing a steel workpiece which is subjected to a heat treatment imparting physical properties to secure a microstructure capable of improving various mechanical properties such as toughness and yield ratio.

Steel used in various structures is considered to be an important property in addition to high strength for light weight and high performance. Steel structures can cause collapse of structures by earthquakes and strong winds, so steel materials with excellent seismic resistance are required according to the use environment. In order to prevent the collapse of the structure due to earthquakes and strong winds, the structural member is composed of steel with high yield strength to support vertical loads, and the seismic member attached to the main structural member is composed of steel with low yield strength. The main member with high yield strength during heavy winds plays a role of maintaining the structure by elastic deformation, and the earthquake and strong wind energy absorb shocks by plastic (reverse) deformation of the earthquake resistant members, thereby improving the seismic resistance of the steel for steel structures. (Japanese Patent Laid-Open No. 96-42187).

For example, it is required to use a bolt having excellent shock resistance with the same concept even in a bolted joint of a steel structure having excellent shock resistance. Since the seismic resistance becomes higher as the yield ratio of steel is lower, it is necessary to make the material of the bolt have the characteristics of resistance yield ratio. In addition, in order to maintain the advantages of the bolted joint according to the high-strength bolt fastening is also required to have a high strength characteristics.

However, if the yield strength of the steel is lowered, the impact absorption amount is too small to expect the effect, and in order to have more impact absorption capacity, it can be seen that it is desirable to achieve resistance strength while achieving further high strength. The advantages of the development of high-strength resistive bolted steel are that the member fastening for the efficient construction of the steel structure, the weight reduction of the bolt, multi-function and high performance are achieved, and the seismic design is also provided at the bolt joint during the seismic design of the steel structure. The earthquake resistance can be further improved.

On the other hand, the shape of the various bolts are usually manufactured by cold forming, high-strength material is required to soften heat treatment before the cold forming high material strength, it is preferable to secure the tensile strength of less than 60kg / mm ² before cold forming. This is to suppress the increase of the die wear rate during cold forming to the maximum. Domestic steel structure fastening bolts are currently capable of cold forming bolts under tensile strength of 60kg / mm ² or less. Therefore, in order to use high-strength bolt material, it is necessary to secure not only excellent delayed fracture resistance but also cold forming required for bolt manufacturing.

Softening heat treatment for securing cold formability is mostly applied by spheroidizing heat treatment, characterized in that the microstructure is composed of ferrite + cementite. However, there is a problem in that the strength of the spheroidized material increases as the amount of alloy element added increases, and there is a limit in the microstructure configuration to overcome this problem.

In the conventional bolt material, the microstructure is mainly characterized by a quasi single phase structure of tempered martensite, in which case the yield ratio is at least 0.8. In order to achieve the resistance ratio, the distribution of soft tissues that can be preferentially deformed during tensile deformation is inevitable in the microstructure, which causes a problem of deterioration in strength. In other words, if the tempering temperature is increased in order to lower the yield ratio, the yield strength is decreased, but at the same time, the tensile strength is also decreased. Therefore, the production of high strength resistive bolt steel for the conventional tempered martensite structure is limited in terms of microstructure configuration characteristics.

Conventional representative techniques for improving the seismic resistance of steel materials include (1) Japanese Unexamined Patent Publication No. 97-95734, (2) Korean Patent Application Nos. 98-52836, 98-52838, and the like.

(1) Japanese Laid-Open Patent Publication No. 97-95734 relates to a method for manufacturing a steel for concrete reinforcement having excellent seismic resistance, and has a low relation with bolt wire, but 0.10-0.40% carbon, 0.05-0.60% silicon, and 0.60 manganese. -2..0%, Aluminum 0.005-0.080%, Boron 0.008% or less, Copper 0.3% or less, Nickel 0.3% or less, Molybdenum 0.1% or less, Steels composed of 0.1% or less yield control yield yield strength 345 MPa, Yield It is referred to as a technology that has a yield ratio of 0.8% while having a height of 1.4% or more and an impact toughness of 27J / cm2 or more.

(2) Republic of Korea Patent Application No. 98-52836 and Republic of Korea Patent Application No. 98-52836, the present inventors control the steel components and manufacturing conditions to control the steel component and manufacturing conditions so that the bolts of the stressed structure has a composite structure of bainite or ferrite + bainite The technology to achieve the following level of resistance ratio is completed.

That is, Korean Patent Application No. 98-52836, which is by weight% of carbon 0.40-0.60%, silicon 2.0-4.0%, manganese 0.1-0.8%, chromium 0.1-0.8%, phosphorus and sulfur 0.01% or less, nitrogen 0.005-0.01 %, Not more than 0.005% of oxygen, in which one, two or more of the group consisting of nickel, boron, vanadium or niobium, molybdenum, titanium, tungsten, copper and cobalt is added to the steel at 20 After heating for more than a minute to form austenite single phase, and quenched to a temperature of Ms + 110 ℃ ~ Ms + 210 ℃ and isothermal heating for bainite transformation to produce a bolt having a bainite structure, the bolt is tensile strength Is 130 ~ 178kg / mm ² and the yield ratio is shown to have a characteristic of 0.33 ~ 0.62.

Korean Patent Application No. 98-52838 also includes a bolt having the same component system as the Korean Patent Application No. 98-52836 from Ac3-[(Ac3-Ac1) /1.3] to Ac3-[(Ac3-Ac1) /5.5] Heated inside to form a composite structure consisting of ferrite + bainite having a ferrite phase fraction of 5-25%, and quenching it to a temperature of Ms + 120 ° C to Ms + 180 ° C, followed by isothermal heat treatment to produce a composite of ferrite + bainite. It is manufactured to have a structure, the bolt produced through this technique has a tensile strength of 125 ~ 169 kg / mm ² , the yield ratio is shown to have a characteristic of 0.33 ~ 0.61.

The above-mentioned Korean Patent Application No. 98-52836 and Korean Patent Application No. 98-52836 of (2) may give the meaning of achieving a resistance ratio in their own way, but mechanical properties such as tensile strength, ductility, and shock absorbency In addition to the technical limitations of not improving the resistance ratio, there is no mention of bolt cold forming.

The present invention has been completed in a series of research processes to overcome the technical limitations of the prior art, the purpose of the graphitization heat treatment to ensure the graphite structure advantageous for cold forming, the elongation, impact toughness to the steel workpiece after molding It is to provide a method for producing a steel workpiece to perform a heat treatment imparting physical properties to secure a microstructure that can improve the overall mechanical properties such as yield ratio.

Steel workpiece manufacturing method of the present invention for achieving the above object, by weight%, carbon 0.65-1.50%, silicon 2.0-4.0%, manganese 0.1-0.8%, phosphorus 0.01% or less, sulfur 0.01% or less, nitrogen 0.002- 0.01%, less than or equal to 0.002% of oxygen, including nickel 0.3-2.0%, boron 0.001-0.003%, vanadium: 0.01 to 0.5%, niobium: 0.01 to 0.5%, molybdenum 0.01-0.5%, titanium 0.01-0.2% Steel composed of one or two or more selected from the group consisting of 0.01-0.5% tungsten and 0.01-0.2% copper, remaining Fe and other impurities, and air-cooled by graphitizing at a temperature of Ac1- (60 ± 30 ° C) Heat treatment process,

Cold forming the heat-treated steel into a steel workpiece,

The steel processed product is heated at a temperature of Ac3 transformation point for 20 minutes or more to form austenite single phase, quenched to a temperature of Ms + 220 ° C to Ms + 300 ° C at a cooling rate of 30 ° C / sec or more, and isothermally treated for at least 60 minutes, and The microstructure is air-cooled to have a composite structure of ferrite + residual austenite, and the phase fraction of brachy type residual austenite in the residual austenite is 10 to 30%.

Hereinafter, the present invention will be described in detail.

In the present invention, the term 'steel workpiece' refers to a workpiece (molded product) formed by processing a steel material (for example, a wire) in a predetermined form, and refers to any workpiece that can use the mechanical properties of the composite steel of the present invention. Include. In particular, the steel of the present invention is a component system useful for manufacturing as a wire rod, and examples thereof include workpieces such as bolts, nuts, and springs that can be processed with the wire rod.

The inventors of the present invention, based on the results that in the cold forming of high-strength bolts, excellent cold formability can be obtained regardless of the presence or absence of alloying elements in the material tensile strength or surface hardness compared to the conventional spheroidization heat treatment method when using the graphitized structure Will be proposed. The present invention is graphitized heat treatment at the temperature of Ac1- (60 ± 30 ℃) before cold forming steel (wire) into a steel workpiece to have a graphite grain size of 50㎛ or less, the graphite grain phase fraction 0.1% or more It is to easily solve the problem of lowering the die wear rate during cold forming.

In addition, the present invention, while metallurgical investigation of the effect of steel microstructure on strength, yield ratio, and other mechanical properties, the residual austenite percentage of brachytype of residual austenite in the composite structure of ferrite + residual austenite When this constant amount, other mechanical properties are also improved while finding that the high strength and the resistance ratio are improved, the present invention has been completed. That is, in the present invention, in the initial stage when the tensile load is applied, the soft structure ferrite starts to deform preferentially, and after the critical deformation amount, the high strength and the resistance ratio are achieved by appropriately inducing plastic organic transformation of the retained austenite. From this point of view, the steel of the present invention and a method of manufacturing the same will be described separately.

[Composite Steel]

Carbon (C): 0.65-1.50%

If the carbon content is less than 0.65%, it is difficult to obtain an adequate amount of retained austenite, shape and size in the ferritic + residual austenite after heat treatment for the production of ferritic + residual austenite composite steel. This is because it is difficult to secure sufficient tensile strength and yield strength as molten steel. In addition, when the carbon content is more than 1.50%, characteristics such as cross-sectional reduction rate, elongation rate and impact toughness decrease after heat treatment, segregation and surface flaws occur during wire rod manufacturing, and surface decarburization deepens when charging furnace, and bolts are fastened. Permanent deformation and static fatigue characteristics deteriorate, the appropriate shape and size of microcomposite tissues, the transformation time required to secure ferrite + residual austenite complex tissues, carbon concentration in residual austenite, and interfacial tool distribution Because it has a bad effect. The preferred carbon content in the present invention is 0.8 to 1.0%, which is more advantageous for the isothermal heat treatment time for producing the ferrite + residual austenite composite structure and the fraction, size and shape of the appropriate residual austenite. In addition, it is possible to improve the strength of the composite structure, high toughness, high elongation, high section reduction rate, delay fracture resistance (diffuse hydrogen amount, precipitation control of grain boundary precipitate), suppress surface decarburization during heat treatment, and bolt fastening This is because post stress relaxation or permanent strain resistance, dynamic and static fatigue characteristics can be improved very effectively.

Silicon (Si): 2.0-4.0%

If the silicon is less than 2.0%, the graphitization heat treatment time for improving the cold forming property is long, and the mechanical and thermal stability of the retained austenite is degraded after the ferrite transformation, resulting in the ferrite + residual austenite composite structure and the amount of retained austenite. And it is difficult to secure the strength due to the insufficient solidification effect of ferrite, and also affects delayed fracture resistance, surface corrosion characteristics, impact toughness, permanent deformation during bolting, and wire rod for controlling decarburization of wire. This is because it is difficult to secure uniformity and proper thickness of the surface ferrite decarburization layer in the furnace, and thus, decarburization is intensified, and it is difficult to control the surface scale characteristics by increasing the hardenability during wire cooling. If the silicon exceeds 4.0%, the above-mentioned effect is saturated and adversely affects the hardenability, the composition of the composite tissue steel, the impact toughness, the fatigue characteristics, and the production of bloom or billet for wire rod manufacturing. This is because the quality characteristics in the final product are deteriorated due to the inhomogeneity of the microstructure due to the segregation of silicon at the same time, and it is difficult to control the homogeneous surface decarburization because the thickness of the surface ferrite layer increases during heat treatment. More preferred silicon component range in the present invention is 2.8-3.3%, isothermal heat treatment time and residual austenite fraction and size, shape, ferrite + residual austenite composite structure for producing bainite structure (ferrite + residual austenite) Strength, high toughness, delayed fracture resistance (diffuse hydrogen content, precipitation control of grain boundary precipitate), surface decarburization, stress relaxation or permanent deformation resistance after bolting, and dynamic and static fatigue characteristics This can be effectively improved.

Manganese (Mn): 0.1% to 0.8%

Manganese (Mn) is an element that forms a solid solution to form a solid to form a solid solution, and is very soluble in high-strength bolt characteristics. Therefore, its content is detrimental to the strength of the base metal, hardenability during heat treatment, stress relaxation, and segregation. It is preferable to set it as 0.1-0.8% in consideration of these. If the content of manganese exceeds 0.8%, local quenchability due to manganese segregation is increased rather than solid solution strengthening effect. Because it is crazy.

Phosphorus (P) and sulfur (S): 0.01% or less each

Phosphorus segregates at grain boundaries and degrades toughness, limiting its upper limit to 0.01%. Sulfur is a low melting point element, and segregates grains to reduce toughness and form an emulsion, which has a detrimental effect on delayed fracture resistance and stress relaxation characteristics. The upper limit is limited to 0.01%.

Nitrogen (N): 0.002-0.01%

If the nitrogen content is less than 0.002%, it is difficult to form vanadium and niobium-based nitrides that act as non-diffusion hydrogen trap sites, and if the content exceeds 0.01%, the effect is saturated.

Oxygen (O): 0.002% or less

This is because when the content of oxygen exceeds 0.0020%, coarse oxide-based nonmetallic inclusions are easily formed and fatigue life is reduced.

In the above composition, nickel 0.3-2.0%, boron 0.001-0.003%, vanadium: 0.01-0.5%, niobium: 0.01-0.5%, molybdenum 0.01-0.5%, titanium 0.01-0.2%, tungsten 0.01-0.5%, One or more selected from the group of 0.01-0.2% copper

Vanadium (V) or niobium (Nb): 0.01% to 0.5%

Vanadium (V) or niobium (Nb) is an element that improves delayed fracture resistance and stress relaxation. If the content is less than 0.01%, the distribution of vanadium or niobium-based precipitates in the base material decreases, and thus the role of the non-diffusible hydrogen trap site is insufficient. Therefore, it is difficult to expect the effect of improving the delayed fracture resistance and the precipitation strengthening is expected. This is because it is difficult to improve the stress relaxation resistance is not sufficient, and it is difficult to expect attenuation of austenite grains, which affects the microstructure of the ferrite + residual austenite composite. In addition, if it exceeds 0.5%, the graphitization heat treatment time is long, and the improvement effect on delayed fracture resistance and stress relaxation resistance by V or Nb-based precipitates is saturated and coarse that is not dissolved in the base material during austenite heat treatment. This is because the amount of alloy carbide increases and acts like a non-metallic inclusion, leading to a decrease in fatigue properties.

Nickel (Ni): 0.3 to 2.0%

Nickel (Ni) is an element for promoting graphitization and is an element that improves delayed fracture resistance by forming a nickel enriched layer on the surface during heat treatment to suppress permeation of external hydrogen. If the nickel content is less than 0.3%, the formation of the surface thickening layer is incomplete, so it is difficult to expect the effect of improving the delayed fracture resistance, and the heat treatment time is increased during the spheroidization or graphitization treatment to improve the decarburization control, toughness and cold forming. This is because there is no improvement effect of cold forming during bolt forming. If it exceeds 2.0%, the effect is saturated and negatively affects the appropriate amount, size and shape of the amount of retained austenite.

Boron (B, B): 0.001% to 0.003%

Boron (boron, B) is a graphitization promoting element in the present invention, and is a grain boundary strengthening element for improving quenchability and delayed fracture resistance. If the content of boron is less than 0.0010%, the effect of improving grain boundary strength due to grain boundary strengthening of boron atoms due to grain boundary segregation during heat treatment is insufficient, and the graphitization promoting effect is insufficient during graphitization treatment to improve cold forming. If the content of boron exceeds 0.003%, the effect is saturated, rather, the precipitation of boron nitride at the grain boundary leads to a decrease in grain boundary strength.

Molybdenum (Mo) and Tungsten (W): 0.01-0.5% each

If the content is less than 0.01%, the grain boundary strengthening effect of the ferrite and the retained austenite is insufficient, and the hardenability during the heat treatment, the solid solution strengthening of the ferrite, and the Mo and W system precipitation strengthening effects are insufficient. If it exceeds 0.5%, the effect is saturated, and as the hardenability is increased, it is easy to form low-temperature structure (martensite + bainite) during wire rod manufacturing, and the heat treatment time during spheroidization or graphitization treatment to improve cold formability This is because there is a disadvantage.

Copper (Cu): 0.01-0.2%

If the copper content is less than 0.01%, the effect of promoting graphitization and improvement of corrosion resistance is insufficient. If the copper content is more than 0.2%, the improvement effect is saturated, and the melting point is lowered during the grain boundary segregation. This is because surface flaw is more likely to occur due to grain embrittlement during charging, and impact toughness of the final product is lowered.

Titanium: 0.01-0.2%

If the content of titanium is less than 0.01%, the effect of promoting graphitization and miniaturizing austenite grains is insufficient, and the precipitation distribution of titanium-based carbon and nitride in the grain boundary effective for delayed fracture resistance is insufficient, so that improvement effect is difficult to expect. This is because when it exceeds 0.2%, the additive effect is saturated, and coarse titanium-based carbon and nitride are formed to affect the mechanical properties.

The steel (wire) formed as described above is subjected to (1) graphitization heat treatment, (2) cold forming, and (3) physical property imparting heat treatment.

(1) graphitization heat treatment

In the present invention, the wire rod formed as described above is graphitized. Of course, the wire rod is made of a wire rod rolled ingot (bloom or billet) as in the usual method. In the present invention, it is preferable that the graphitization heat treatment is performed to obtain a graphite grain size of 50 µm or less and its phase fraction of 0.1% or more in the wire rod. The graphitization heat treatment may be applied to a process of cooling the billet by hot wire rolling or may perform a separate heat treatment. Wire rods are usually drawn, and spheroidizing heat treatment is performed before and after the drawing. Therefore, when the graphitization heat treatment is performed instead of the spheroidization heat treatment, the cold forming of the wire rod is ensured, so that the spheroidization heat treatment can be omitted.

Graphitization heat treatment is preferably performed at Ac1- (60 ± 30 ° C) for 5 minutes or more. If the graphitization heat treatment temperature is lower than Ac1-90 ° C, the graphitization rate is very slow, which is a problem in securing productivity.At the temperature of Ac1-30 ° C, the graphitization heat treatment time is long and the austenite phase is precipitated. This is because it is highly likely to be reused. This is because the graphitization is difficult to be completely performed at the heat treatment time of 5 minutes or less.

(2) cold forming process

As described above, the new wire material having an appropriate graphite structure is excellent in cold formability. Therefore, cold forming is performed on steel products such as bolts, springs and nuts.

(3) Property grant heat treatment

The cold formed steel workpiece (bolt) is heat treated to obtain a steel workpiece having the desired mechanical properties. In the present invention, the ferrite + residual austenite composite structure in the present invention through the heat treatment, the heat treatment conditions are as follows.

First, the processed bolts are heated at a temperature above the Ac 3 transformation point. This is because it is difficult to secure the austenite single phase as an abnormal region where the ferrite and the austenite phase coexist below the Ac 3 transformation point, and may cause a heterogeneity of the tissue. This heating temperature is below 1150 ℃. If it exceeds 1150 ℃, it causes surface decarburization and coarsening of austenite grains when heating the material, so that the quality characteristics (mechanical properties, stress relaxation, surface scratches, static fatigue) in the final product Characteristics, etc.). The heating time is heated by the time necessary for the austenitization to occur, but it is possible if it is about 20 minutes or more.

By heating as described above, it is preferable to cool the steel workpiece to a temperature of Ms + 220 ° C to Ms + 300 ° C (Ms is the martensite transformation start temperature) at a cooling rate of 30 ° C / sec or more. This is not preferable at less than Ms + 220 ℃ because the mechanical strength of the residual austenite is increased by affecting the amount, size and shape of the residual austenite constituting the composite structure at constant temperature transformation, which increases the yield strength. In addition, when the Ms + 300 ℃ exceeds the perlite transformation occurs because it is difficult to secure a complex structure consisting of the ferrite + residual austenite pursued in the present invention.

The isothermal heat treatment is performed for 60 minutes or more in the above-described temperature section, followed by oil-cooling or air-cooling, so that the phase fraction of the brachy-type residual austenite in the residual austenite is 10-30% in the composite structure of ferrite + residual austenite.

The steel formed as described above has a microstructure of ferrite + residual austenite composite structure, and the blocky-type residual austenite phase percentage of the total residual austenite percentage in the composite structure is in the range of 10 to 30%. It is desirable to. The reason for this is that if the residual austenite phase fraction of the brachy type is less than 10%, the carbon concentration in the retained austenite is increased due to the carbon employment limit in the ferrite, thereby increasing the mechanical stability of the retained austenite. This is because the plastic organic transformation of the retained austenite cannot be induced and thus a low yield strength cannot be secured. In addition, when the brachy type austenite phase fraction is more than 30%, there is no problem in securing a resistance ratio, but it is not preferable because the elongation, cross-sectional reduction rate, and impact toughness are sharply lowered.

Hereinafter, the present invention will be described in detail by way of examples.

Example 1

Steels having the composition as shown in Table 1 were cast as 50 kg ingot, and then homogenized and heat-treated at 1250 ° C. for 48 hours to obtain hot wires with a thickness of 13 mm. At this time, the finishing temperature was 950 ℃ or more and hot-rolled after hot rolling, the rolling ratio was 80% or more.

Chemical composition C Si Mn Cr V Ni Mo Ti W B Cu P S N2 O2 Invention steel One 0.80 3.06 0.53 - 0.05 - - - - - 0.03 0.009 0.009 0.004 0.0013 2 0.70 3.44 0.52 - 0.06 - - 0.02 - 0.001 0.04 0.007 0.008 0.004 0.0014 3 0.89 3.13 0.58 - 0.07 0.72 - - 0.06 - 0.12 0.006 0.009 0.005 0.0015 4 0.85 2.25 0.87 - Nb: 0.01 - 0.23 0.04 0.15 - 0.03 0.006 0.009 0.004 0.0016 5 0.83 3.90 0.55 - 0.04 - - - 0.0020 0.03 0.007 0.006 0.005 0.0017 6 1.24 3.22 0.64 - - - 0.05 0.03 0.07 0.0023 0.02 0.007 0.007 0.005 0.0017 7 1.39 2.57 0.82 - - 1.20 0.20 0.09 - - 0.15 0.009 0.008 0.005 0.0018 Comparative steel One 0.81 2.93 0.33 0.49 0.04 - - - - - - 0.007 0.009 0.006 0.0012 2 0.68 3.54 0.35 0.74 0.19 - - 0.01 - 0.001 - 0.009 0.006 0.012 0.0015 3 0.90 3.04 0.29 0.38 0.06 0.66 - - 0.03 - - 0.004 0.008 0.008 0.0017 4 0.83 2.09 0.71 0.55 0.12 - 0.25 0.03 0.18 - - 0.005 0.004 0.011 0.0013 5 0.82 3.92 0.32 0.32 0.05 - - - 0.0019 - 0.006 0.004 0.008 0.0015 6 1.21 3.11 0.30 0.56 - - 0.04 0.05 0.09 0.0013 - 0.007 0.006 0.005 0.0016 7 1.42 2.61 0.79 0.33 - 1.10 0.10 0.10 - - - 0.009 0.005 0.005 0.0018

The test pieces for evaluating mechanical properties (tensile and impact characteristics) and delayed fracture resistance from the hot rolled materials as described above were taken in the rolling direction of the rolled material.

The graphitization heat treatment for showing the effect of the present invention was air cooled after maintaining for 10 hours at 750C, spheroidization heat treatment of the comparative steels after cooling for 5 hours at 830C to 650C after cooling slowly at 10C / hr cooling rate.

The wire made of the inventive steel (1-7) of Table 1 is heated for 20-40 minutes at the range of 1150 ° C. or below at the Ac3 transformation point, which is the austenitic single phase inverse heating temperature range, and isothermal to obtain a ferrite + residual austenite composite structure. In the heat treatment temperature range, Ms + 220 ° C. to Ms + 300 ° C. were quenched at a cooling rate of 30 ° C./sec or more, and heat-treated for 60 to 100 minutes to prepare a phase ratio of Brachy type residual austenite in the range of 18 to 33%. 3 (1-11)).

 Isothermal heat treatment for heating the wire made of comparative steel (1-7) of Table 1 for 20-40 minutes in the range of Ac3 transformation point, which is the austenite single phase inverse heating temperature range, below 1150 ° C, to obtain ferrite + residual austenite composite structure. In the temperature range, Ms + 240 ° C to Ms + 290 ° C, quenching was performed at a cooling rate of 30 ° C / sec or more and heat-treated for 60 minutes, thereby preparing a phase percentage of Brachy type residual austenite in a range of 15 to 30% (Invention of Table 2). Ash (1-11)).

In order to evaluate the tensile and impact characteristics of the materials manufactured as described above, the tensile test piece was tested at a cross head speed of 5 mm / min using the KS standard (KS B 0801) No. 4 test piece. The impact test piece was manufactured according to KS standard (KS B 0809) No. 3 test piece, and the notch direction was machined from the side of the rolling direction (L-T direction).

In addition, the delayed fracture resistance evaluation for showing the effect of the present invention was applied to the constant load method commonly used. This evaluation method is a general method of evaluating the delayed fracture resistance by the time required for additional stress or the failure under a specific stress. In the delayed fracture test, the test stress was determined based on the notched tensile strength. The delayed fracture tester used a constant loading type delayed fracture testing machine. The delayed fracture test specimen was prepared with a specimen diameter of 6 mmφ, a notch diameter of 4 mmφ, and a notch root radius of 0.1 mm. The test piece atmosphere solution was performed at room temperature (25 ± 5C) at a pH of 2 ± 0.5 with a wolpole buffer solution (HCl + CH3COONa).

In addition, the critical delayed fracture stress ratio means the load stress / notch tensile strength ratio in which the time required to break is not broken up to 150 hours or more, and the notch strength is the value of (maximum load ÷ notch section area) by tensile test of the notched test piece. Was obtained. The number of test pieces for setting the critical delay fracture strength was based on the case where more than 13 undecided specimens were used.

After the heat treatment, the residual austenite phase fraction in the microcomposite was measured by X-ray (Cu radiation), and the blocky-type residual austenite phase percentage for the effect of the present invention was point counting, which is a general optical microscope measurement method. couting) and the test surface was 1000mm ² . Tensile properties and impact toughness of the inventive and comparative materials prepared as described above were measured, and the results are shown in Table 4.

division Graphite Grain Percentage (%) Mechanical properties after graphitization or spheroidization heat treatment Tensile Strength (kg / mm ² ) Yield strength (kg / mm ² ) Elongation (%) Cross section reduction rate (%) Impact Toughness (J / ㎠) Inventive Steel 1 2.4 59 41 30 52 56 Inventive Steel 2 2.3 55 39 35 60 59 Invention Steel 3 2.6 60 42 27 40 53 Inventive Steel 4 2.2 55 37 36 64 65 Inventive Steel 5 2.5 59 38 29 58 58 Inventive Steel 6 2.7 57 35 37 66 68 Inventive Steel 7 2.4 60 43 31 62 64 Comparative Steel 1 - 78 57 32 53 62 Comparative Steel 2 - 77 58 33 56 64 Comparative Steel 3 - 81 63 29 58 58 Comparative Steel 4 - 83 65 27 63 54 Comparative Steel 5 - 79 62 30 59 60 Comparative Steel 6 - 84 63 25 53 57 Comparative Steel 7 - 88 65 26 63 54

division Mechanical Properties After Isothermal Heat Treatment Tensile Strength (kg / mm ² ) Yield strength (kg / mm ² ) Yield Ratio (YS / TS ratio) Elongation (%) Impact Toughness (J / ㎠) Critical Delay Failure Stress Ratio                                              Invention 1 183 65 0.36 25 95 0.35 Invention 2 185 60 0.32 26 90 0.3 Invention 3 192 44 0.23 24 85 0.3 Invention 4 191 57 0.32 22 80 0.35 Invention 5 180 63 0.30 25 80 0.3 Invention 6 195 64 0.33 26 95 0.35 Invention 7 201 63 0.31 21 94 0.3 Invention Material 8 190 60 0.32 22 84 0.3 Invention 9 186 41 0.22 20 81 0.3 Invention 10 185 62 0.34 25 89 0.35 Invention 11 193 65 0.34 23 105 0.3 Comparative Material 1 186 70 0.37 28 90 0.3 Comparative material 2 189 65 0.34 25 85 0.35 Comparative material 3 190 48 0.25 23 80 0.3 Comparative material 4 190 62 0.32 24 85 0.35 Comparative material 5 183 69 0.38 23 80 0.3 Comparative Material 6 197 72 0.37 29 100 0.3 Comparative material 7 200 74 0.37 25 90 0.3 Comparative Material 8 195 66 0.34 25 80 0.3 Comparative material 9 190 45 0.24 21 80 0.3 Comparative Material 10 187 75 0.40 24 85 0.35 Comparative Material 11 194 79 0.41 20 80 0.35

Table 2 is a result of evaluating the mechanical properties after the graphitization heat treatment for the inventive steels of Table 1. As shown in Table 2, the tensile strength of the inventive steels ranges from 55 to 60 kg / mm ² , but the comparative steels softened by spheroidizing heat treatment have a tensile strength of 17 kg / mm to 88 kg / mm 2. As can be seen that it can be reduced by more than ² mm, the present invention can expect a significant improvement in terms of bolt cold forming

As shown in Table 4, the invention material (1-11) are the yield ratio of 0.22 ~ 0.36, tensile strength 183 - As shown the 201kg / mm ² elongation range 20 ~ 28%, the impact toughness of 80 ~ 105J / cm ^2, the threshold delay The fracture stress ratio ranged from 0.3 to 0.35.

On the other hand, the comparative materials (1-11) showed yield ratio 0.25 ~ 0.41, tensile strength 183 ~ 200kg / mm ² , elongation 20 ~ 29%, impact toughness 80 ~ 100J / cm2, critical delay fracture stress ratio 0.3 ~ 0.35 As can be seen from the range, the present invention exhibits an equivalent level of mechanical properties and critical rupture stress ratios compared to the comparative materials.

As can be seen from these results, by providing a graphitization heat treatment in the manufacture of high-strength bolts, solving the problem of the cold forming, and a complex structure of ferrite + residual austenite and the residual austenite of the brachy type of residual austenite is 10-30 It is understood that the present invention steels, which are%, can secure elongation and impact toughness while having excellent strength and yield ratio of equivalent levels or more compared to conventional steel grades.

As described above, the present invention provides an alloy composition system and heat treatment condition of a composite steel structure composed of ferritic + residual austenite, which has excellent cold forming property, high strength, and strength ratio, and has high elongation and high impact toughness. By presenting, it is possible to provide a high strength resistance composite composite steel having excellent elongation and impact toughness by achieving high strength while simultaneously achieving a high resistance ratio.

Claims (5)

By weight, it contains carbon 0.65-1.50%, silicon 2.0-4.0%, manganese 0.1-0.8%, phosphorus 0.01% or less, sulfur 0.01% or less, nitrogen 0.002-0.01%, oxygen 0.002% or less, and nickel 0.3 -2.0%, boron 0.001-0.003%, vanadium: 0.01-0.5%, niobium: 0.01-0.5%, molybdenum 0.01-0.5%, titanium 0.01-0.2%, tungsten 0.01-0.5%, copper 0.01-0.2% Heat treatment step of air cooling by graphitizing one or two or more selected from among steels composed of remaining Fe and other impurities at a temperature of Ac1- (60 ± 30 ° C.), Cold forming the heat-treated steel into a steel workpiece, The steel workpiece is heated at a temperature above Ac3 transformation point for more than 20 minutes to form austenite single phase, quenched to a temperature of Ms + 220 ° C to Ms + 300 ° C at a cooling rate of 30 ° C / sec or more, and isothermally treated for at least 50 minutes and Air-cooled, the microstructure has a composite structure of ferrite + residual austenite, and the seismic type high carbon steel processed product comprising the phase fraction of brachy-type residual austenite in the residual austenite is 10 to 30% Manufacturing method. The method of claim 1, wherein the carbon is contained in the range of 0.8% to 1.0%. The method of claim 1, wherein the silicon is contained in a range of 2.8 to 3.3%. The method of claim 1, wherein the phase fraction of the brachy-type residual austenite is 15 to 25%. The method of claim 1, wherein the cold forming is performed by forming steel into bolts.