KR20010060700A - High strength duplex steel having a superior elongation percentage, impact strength and low yield strength and bolt made of the steel and method for manufacturing working product by using it - Google Patents

Abstract

PURPOSE: A process for preparing a steel composition capable of improving mechanical characteristics such as tensile strength, elongation, impact toughness or the like while maximizing low yield ratio of steel is provided, which improves mechanical characteristics while controlling a blocky type phase fraction of retained austenite in a composite structure of ferrite and retained austenite. CONSTITUTION: This steel of composite structure comprises by weight, C:0.65 to 1.50%, Si:2.0 to 4.0%, Mn:0.1 to 0.8%, Cr:0.1 to 0.8%, P:0.01% or less, S:0.01% or less, N:0.005 to 0.01%, O:0.05% or less, one or two or more selected from Ni:0.3 to 2.0%, B:0.001 to 0.003%, V:0.01 to 0.5%, Nb:0.01 to 0.5%, Mo:0.01 to 0.5%, Ti:0.01 to 0.2%, W:0.01 to 0.5% and Cu:0.01 to 0.2%. The fine structure of the steel is a composite structure of ferrite and retained austenite and a retained austenite phase fraction of blocky type of retained austenite is 20 to 35%.

Description

High strength duplex steel having a superior elongation percentage, impact strength and low yield strength and bolt made of the steel and method for manufacturing working product by using it}

The present invention relates to a steel with high strength and low yield ratio, and more specifically, to a composite structure of ferrite and residual austenite, by controlling the phase ratio of the blocky type in the residual austenite of the composite structure, elongation, impact It relates to a bolt made of high strength steel with improved mechanical properties such as toughness, yield ratio, etc., and a method of manufacturing the steel as a steel workpiece.

Steel used in various structures is considered to be an important property with high strength and high strength for light weight and high performance. Steel structures can cause the collapse of structures due to earthquakes and strong winds, so steel materials with excellent earthquake resistance are required according to the environment. Structural members are composed of steel with high yield strength to prevent the collapse of structures by earthquakes and strong winds. The seismic member supporting the vertical load and attached to the main structural member is composed of steel with low yield strength, so that the main member with high yield strength during earthquake and strong wind maintains the structure by elastic deformation. The energy of the strong wind absorbs the impact by the plastic (reverse) deformation of the seismic member, 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. Seismic resistance of steel It is necessary to ensure that the material of the bolt has the properties of the resistive ratio because the lower the larger the value. 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.

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, the yield ratio ( Increasing the tempering temperature to decrease) decreases the yield strength but also decreases the tensile strength. 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 has a meaning as a technology to secure a yield ratio of 0.8% while having a height of 1.4% or more and an impact toughness of 27J / cm ² or above.

(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, wherein one or 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 temperatures above the Ac ₃ transformation point. After heating for more than 20 minutes to form austenite single phase, quench it 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.

Republic of Korea Patent Application No. 98-52838 The bolt having the same component system as the Republic of Korea Patent Application No. 98-52836 described above Ac ₃ -[(Ac _3- Ac ₁ ) / 1.3] Ac ₃ -[(Ac _3- Ac ₁ ) /5.5] is made into a composite structure consisting of ferrite + bainite having a ferrite phase fraction of 5-25%, and then quenched to a temperature of Ms + 120 ° C to Ms + 180 ° C and isothermally treated. It is manufactured to have a composite structure of ferrite + bainite, and the bolt manufactured through this technique has a tensile strength of 125 to 169 kg / mm ² and a yield ratio of 0.33 to 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, there is a technical limitation that it is not improving the resistance ratio.

The present invention has been completed in a series of studies to overcome the technical limitations of the prior art, the purpose of which is to improve the mechanical properties such as tensile strength, elongation, impact toughness while maximizing the steel's resistance ratio It provides a material and a method of manufacturing the same. This invention can impart the meaning that the mechanical properties are improved by controlling the phase fraction of the brachy type in the residual austenite in the composite structure of ferrite + residual austenite.

Steel 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%, chromium 0.1-0.8%, phosphorus and sulfur 0.01% or less, nitrogen 0.005-0.01 %, 0.005% or less of oxygen, including 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%, One or two or more selected from the group consisting of tungsten 0.01-0.5%, copper 0.01-0.2%, the remaining Fe and other impurities, the microstructure has a complex structure of ferrite + residual austenite, the residual austenite Among the knights, the brachy type includes one having a 10 to 30% phase fraction.

In addition, the manufacturing method of the steel workpiece of the present invention, carbon by weight 0.65-1.50%, silicon 2.0-4.0%, manganese 0.1-0.8%, chromium 0.1-0.8%, phosphorus 0.01% or less, sulfur 0.01% or less, nitrogen 0.005-0.01%, oxygen 0.005% or less, including 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- Austenitic steels containing one or two or more selected from the group consisting of 0.2%, 0.01-0.5% tungsten and 0.01-0.2% copper and composed of the remaining Fe and other impurities are heated at a temperature above the Ac ₃ transformation point. It is made into a single phase, quenched to a temperature of Ms + 220 ° C to Ms + 300 ° C at a cooling rate of 30 ° C / sec or more, isothermally heat treated for 60 minutes or more, and cooled or air-cooled to form a complex structure of ferrite + residual austenite. It is comprised, including so that the phase ratio of the brachy type in a knight may be 10-30%.

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 present inventors have studied the effect of steel microstructure on strength, yield ratio, and other mechanical properties, while the metallurgical study of ferrite + residual austenite has a certain amount of brachytype residual austenite in the residual austenite. When this was achieved, the present invention was found to improve the high strength and the resistive ratio while improving other mechanical properties. 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. Here, the brachy type refers to a shape in which the ratio of the abscissa and the longitudinal axis is about 0.5 or more.

[Composite Steel]

It is preferable to make content of carbon (C) into 0.65-1.5%. 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.

It is preferable to limit the content of silicon (Si) to 2.0-4.0%. If the silicon is less than 2.0%, the mechanical and thermal stability of the retained austenite after ferrite transformation decreases, making it difficult to secure the ferrite + residual austenite composite structure and the appropriate amount of retained austenite. Difficulties also affect delayed fracture resistance, surface corrosion characteristics, impact toughness, permanent deformation during bolt fastening, and ensure uniformity and proper thickness of the surface ferrite decarburized layer in the wire heating furnace for wire decarburization control. This is because it is difficult to perform decarburization, and it is difficult to control surface scale characteristics due to increased hardenability during wire rod 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 the 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, high strength and high toughness, delayed fracture resistance (diffusive hydrogen content, grain boundary) for producing bainite structure (ferrite + residual austenite) Precipitation control), surface decarburization, stress relaxation after bolting or permanent deformation resistance, dynamic and static fatigue characteristics can be improved very effectively.

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. 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. That is, macro sedimentation and micro segregation are easy to occur depending on the segregation mechanism during steel coagulation. Manganese segregation promotes segregation zone due to relatively low diffusion coefficient compared with other elements, which improves hardenability, which results in low temperature structure of wire cores. core martensite). In addition, when less than 0.1% of manganese is added, the formation of segregation zones due to manganese segregation is hardly achieved, but the improvement of hardenability, permanent strain resistance and stress relaxation is difficult to expect due to the lack of solid solution strengthening effect.

It is preferable to make content of chromium (Cr) into 0.1 to 0.8%. If the content of chromium is less than 0.1%, it is difficult to form a surface ferrite layer for surface decarburization control during heat treatment of high silicon-added steel, so that there is almost no decarburization inhibitory effect, and it is difficult to expect quenchability improvement. In addition, if it exceeds 0.8%, it is not preferable because the time required for transformation of ferrite + residual austenite composite structure becomes longer during isothermal heat treatment, and it is difficult to generate a proper surface ferrite layer when charging the wire furnace for wire rod decarburization layer control. This is because it affects decarburization control.

The content of phosphorus (P) and sulfur (S) is preferably 0.01% or less. Phosphorus segregates at grain boundaries and degrades its toughness, limiting its upper limit to 0.01%. Sulfur is a low melting point element that segregates grain boundaries 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%.

The content of nitrogen (N) is preferably made 0.005-0.01%. This is because when the nitrogen content is less than 0.005%, it is difficult to form vanadium and niobium-based nitrides that act as non-diffusion hydrogen trap sites, and when the content exceeds 0.01%, the effect is saturated.

The content of oxygen (O) is preferably made 0.0015% or less. This is because when the oxygen content exceeds 0.0015%, coarse oxide-based nonmetallic inclusions are easily formed and fatigue life is reduced.

Vanadium (V) or niobium (Nb) is an element for improving delayed fracture resistance and stress relaxation resistance, and the addition thereof is preferably 0.01% to 0.5% for one kind or two kinds. If the content is less than 0.05%, 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. 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, when it exceeds 0.5%, the improvement effect on delayed fracture resistance and stress relaxation resistance by V or Nb-based precipitates is saturated, and the amount of coarse alloy carbides which are not dissolved in the base metal during austenite heat treatment increases, such as nonmetallic inclusions. This is because it causes a decrease in fatigue characteristics.

Nickel (Ni) is an element that improves the delayed fracture resistance by forming a nickel thickened layer on the surface during heat treatment to suppress permeation of external hydrogen, and the content thereof is preferably 0.3-2.0%. 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 (boron, B) is a grain boundary strengthening element for improving the hardenability and delayed fracture resistance in the present invention, the content is preferably 0.0010 to 0.003%. 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.

The content of molybdenum (Mo) and tungsten (W) is preferably 0.01-0.5%. 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.

The content of copper (Cu) is preferably made 0.01-0.2%. When the copper content is less than 0.01%, the improvement effect on the corrosion resistance is insufficient. When the copper content is over 0.2%, the improvement effect is saturated and the melting point is lowered at the grain boundary segregation. This is because surface flaws are more likely to occur due to grain boundary embrittlement and impact toughness in the final product is lowered.

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

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, which increases 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.

[Method for Manufacturing Steel Work]

The steel prepared as described above is manufactured from any processed material, and then heat-treated so that the final microstructure is ferrite + residual austenite such that the phase fraction of the brachy-type austenite in the residual austenite becomes 10-30%. Isothermal heat treatment). Here, the manufacturing method of a steel workpiece is demonstrated using a bolt as an example.

The bolt is rolled ingot (bloom or billet) into a wire rod and processed into a certain shape to manufacture the bolt. In the present invention, there is a feature in heat treating a bolt processed according to a conventional process.

First, the processed bolt is heated at a temperature above the Ac ₃ transformation point, because below the Ac ₃ transformation point, it is difficult to secure an austenite single phase as an abnormal region where the ferrite and austenite phase coexist 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.

It is preferable to heat as mentioned above to make the structure of the bolt into an austenite single phase, and to cool 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 carried out 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 to 30%.

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.

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.

division C Si Mn Cr Ni B V Mo Ti W P S N Inventive Steel 1 0.81 2.93 0.33 0.49 - - 0.04 - - - 0.007 0.009 0.006 Inventive Steel 2 0.68 3.54 0.35 0.74 - 0.0010 0.19 - 0.01 - 0.009 0.006 0.012 Invention Steel 3 0.90 3.04 0.29 0.38 0.66 - 0.06 - - 0.03 0.004 0.008 0.008 Inventive Steel 4 0.83 2.09 0.71 0.55 - - 0.12 0.25 0.03 - 0.005 0.004 0.011 Inventive Steel 5 0.82 3.92 0.32 0.32 - 0.0019 0.05 - - 0.18 0.006 0.004 0.008 Inventive Steel 6 1.21 3.11 0.30 0.56 - 0.0013 - 0.04 0.05 0.09 0.007 0.006 0.005 Inventive Steel 7 1.42 2.61 0.79 0.33 1.10 - - 0.10 0.10 - 0.009 0.005 0.005 Comparative Steel 1 0.45 3.03 0.29 0.58 - - 0.05 - - - 0.005 0.004 0.008 Comparative Steel 2 0.40 3.42 0.31 0.79 - 0.0013 0.2 - 0.01 - 0.006 0.005 0.014 Comparative Steel 3 0.60 2.99 0.32 0.33 0.54 - 0.05 - - 0.02 0.007 0.009 0.007 Comparative Steel 4 0.45 2.0 0.77 0.51 - - 0.11 0.2 0.03 - 0.006 0.008 0.009 Comparative Steel 5 0.44 3.96 0.23 0.27 - 0.0015 0.06 - - 0.2 0.008 0.008 0.008 Comparative Steel 6 0.53 3.01 0.35 0.55 - 0.0010 - 0.05 0.05 0.07 0.004 0.009 0.004 Comparative Steel 7 0.58 2.56 0.80 0.29 1.10 - - 0.13 0.10 - 0.005 0.006 0.005 Comparative Steel 8 0.35 0.19 0.67 0.95 0.03 - tr 0.17 - - 0.019 0.015 0.004 Comparative Steel 9 0.31 0.20 0.62 0.95 0.04 - tr 0.05 - - 0.017 0.010 0.005 Comparative Steel 10 0.34 0.22 0.36 1.26 0.05 - 0.019 0.40 - - 0.011 0.012 0.015 Comparative Steel 11 0.20 0.20 0.80 0.72 - 0.0015 - 0.04 - - 0.009 0.004 0.005 Inventive steel 2 contains 0.01% of Nb. Inventive steel 3 contains 0.01% Cu. tr is short for trace.

In order to obtain a ferrite + residual austenite composite structure, the wire formed of the inventive steel (1-7) of Table 1 was heated for 20 to 40 minutes at a temperature of 1150 ° C. or lower at the Ac ₃ transformation point, which is the austenitic single-phase reverse heating temperature range. The phase fraction of Brachy type residual austenite was prepared in the range of 15 to 28% by quenching at a cooling rate of 30 ° C./sec or more to Ms + 200 ° C. to Ms + 300 ° C., which is an isothermal heat treatment temperature range, for 60 minutes (Table 2). Invention (1-11)).

In addition, the comparative steel (1-4) of Table 1 is heated for 30-40 minutes in the range of 950-1030 ℃, the austenitic single-phase heating temperature range, Ms at Ms + 160 ℃ isothermal heat treatment temperature range for bainite transformation It was quenched at a cooling rate of 70 ° C./sec or more to + 210 ° C. and heat-treated for 30-40 minutes to prepare a Brachi-type residual austenite in the range of 7-9% (Table 1-4).

In addition, the ferritic phase fraction was prepared in the range of 5-25% by heating the comparative steel (4-7) at a temperature of Ac ₃ -[(Ac ₃ -Ac ₁ ) / 2] for 30-120 minutes in the ideal temperature range. After quenching at a cooling rate of 70 ° C./sec or more to Ms + 140 ° C. for bainite transformation, and isothermally maintaining for 40 minutes, the residual austenite phase fraction of the brachy type was prepared in the range of 4-7%. 5-8)). In addition, the comparative steel (8-11) was heated in the range of 900-950 ° C, which is the austenite single phase region, and tempered at 450 ° C after oil cooling to prepare a conventional tempered martensite structure (Comparative Material (9-12)).

The Ms transformation temperatures of these steels were measured using dilatometry and the results are shown in Table 2. In addition, 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. It was. 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). After the heat treatment, the residual austenite phase fraction in the microcomposite was measured using X-ray (Cu radiation). couting), and the test surface at this time was 1000 mm 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 3.

division Target steel grade Heating temperature (℃) Heating time (min) Isothermal heating temperature (℃) Isothermal heating time (min) Phase fraction of retained austenite (%) Transformation temperature (℃) Ac ₃ Ac ₁ Ms Invention 1 Inventive Steel 1 980 30 Ms + 230 60 15 - - 210 Invention 2 980 30 Ms + 250 60 20 - - Invention 3 980 30 Ms + 290 60 25 - - Invention 4 1050 20 Ms + 250 60 21 - - Invention 5 1150 20 Ms + 220 60 22 - - Invention 6 Inventive Steel 2 1050 40 Ms + 250 60 14 - - 224 Invention 7 Invention Steel 3 1050 40 Ms + 250 90 27 - - 193 Invention Material 8 Inventive Steel 4 1050 40 Ms + 240 90 17 - - 177 Invention Material 9 Inventive Steel 5 1000 40 Ms + 260 90 25 - - 214 Invention 10 Inventive Steel 6 1150 40 Ms + 250 90 28 - - 179 Invention 11 Inventive Steel 7 1100 40 Ms + 230 100 30 - - 140 Comparative Material 1 Comparative Steel 1 950 30 Ms + 210 30 7 - - 290 Comparative Material 2 Comparative Steel 2 1000 40 Ms + 160 40 8 - - 307 Comparative Material 3 Comparative Steel 3 1000 40 Ms + 160 40 9 - - 247 Comparative Material 4 Comparative Steel 4 1030 40 Ms + 160 40 7 - - 278 Comparative Material 5 Comparative Steel 4 X = 2 80 Ms + 140 40 4 880 782 260 Comparative Material 6 Comparative Steel 5 X = 2 30 Ms + 140 40 6 961 842 288 Comparative Material7 Comparative Steel 6 X = 2 40 Ms + 140 40 7 899 817 250 Comparative Material 8 Comparative Steel 7 X = 2 120 Ms + 140 40 5 857 775 208 Comparative Material 9 Comparative Steel 8 900 30 450 ℃ × 60 minutes tempering 0 - - - Comparative Material 10 Comparative Steel 9 900 30 450 ℃ × 60 minutes tempering 0 - - - Comparative Material 11 Comparative Steel 10 950 30 450 ℃ × 60 minutes tempering 0 - - - Comparative Material 12 Comparative Steel 11 900 30 450 ℃ × 60 minutes tempering 0 - - - X is X in the formula Ac3-[(Ac3-Ac1) / X]

division Tensile Strength (kg / mm ² ) Yield strength (kg / mm ² ) Elongation (%) Cross section reduction rate (%) Impact Toughness (J / cm ² ) Yield ratio (yield strength / tensile strength) Invention 1 186 70 28 68 90 0.37 Invention 2 189 65 25 65 85 0.34 Invention 3 190 48 23 60 80 0.25 Invention 4 190 62 24 63 85 0.32 Invention 5 183 69 23 63 80 0.38 Invention 6 197 72 29 70 100 0.37 Invention 7 200 74 25 67 90 0.37 Invention Material 8 195 66 25 65 80 0.34 Invention Material 9 190 45 21 62 80 0.24 Invention 10 187 75 24 67 85 0.40 Invention 11 194 79 20 60 80 0.41 Comparative Material 1 155 50 15 35 55 0.33 Comparative Material 2 162 75 15 40 60 0.46 Comparative Material 3 174 80 15 42 55 0.46 Comparative Material 4 164 70 16 45 75 0.43 Comparative Material 5 160 93 25 43 38 0.58 Comparative Material 6 158 95 19 45 34 0.60 Comparative Material7 161 96 22 50 40 0.60 Comparative Material 8 169 100 20 44 30 0.59 Comparative Material 9 147 135 15 57 30 0.92 Comparative Material 10 147 129 16 58 20 0.88 Comparative Material 11 148 139 15 57 40 0.94 Comparative Material 12 110 95 15 60 50 0.86

As shown in Table 3, the inventive materials (1-11) exhibited a yield ratio of 0.25 to 0.41, a tensile strength of 183 to 200 kg / mm ² , with an elongation of 20 to 29%, cross section reduction of 60 to 70%, and impact toughness of 80 to It had a range of 100 J / cm ² .

In contrast, the comparative materials (1 to 4) exhibited a yield ratio of 0.33 to 0.60, a tensile strength of 155 to 174 kg / mm ² , an elongation of 15 to 16%, a reduction of section of 35 to 45%, and an impact toughness of 55 to 75 J / cm. ^Had a range of ^two . In the case of comparative materials (5 ~ 8), the yield ratio is 0.58 ~ 0.60, the tensile strength is 158 ~ 169kg / mm ² , and the elongation is 19 ~ 25%, the section reduction is 43 ~ 50%, impact toughness 30 ~ 40J / cm ² It was a level having a range of. In the case of comparative materials (9 ~ 12), yield ratio 0.69 ~ 0.94, tensile strength 110 ~ 147kg / mm ² range, elongation 15 ~ 16%, cross section reduction 57 ~ 60%, impact toughness 20 ~ 50J / cm ² Had a level.

As can be seen from these results, the present invention steels having a composite structure of ferrite + residual austenite and the residual austenite of brachy type in the residual austenite are 10 to 30% have a superior strength and yield ratio compared to the conventional steel grades. Elongation, cross-sectional reduction rate, impact toughness can be remarkably improved.

As described above, the present invention provides the alloy composition system and the thermal condition of the composite tissue steel composed of ferrite + residual austenite having high elongation, high section reduction rate and high impact toughness while having high strength and resistance ratio ratio. By presenting, it is possible to provide a high-strength resistance composite composite steel having excellent elongation and impact toughness by achieving a high-strength at the same time while achieving a high-resistance ratio.

Claims (11)

By weight, it contains carbon 0.65-1.50%, silicon 2.0-4.0%, manganese 0.1-0.8%, chromium 0.1-0.8%, phosphorus 0.01% or less, sulfur 0.01% or less, nitrogen 0.005-0.01%, oxygen 0.005% or less Here, 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- One or two or more selected from the group consisting of 0.2%, the microstructure has a complex structure of ferrite + residual austenite, and the residual percentage of brachy type residual austenite in the residual austenite is 10-30% Composite tissue steel having a high-strength resistance ratio excellent in elongation and impact toughness comprising the thing. The composite tissue steel according to claim 1, wherein the carbon is contained in a range of 0.8 to 1.0%. The composite tissue steel according to claim 1, wherein the silicon is contained in a range of 2.8 to 3.3%. The composite tissue steel according to claim 1, wherein the phase fraction of the brachy type residual austenite is 15 to 25%. Bolts with emphasis of any one of claims 1 to 4 According to claim 5, The bolt has a yield ratio of 0.25 ~ 0.41, tensile strength 183 ~ 200kg / mm ² , elongation 20 ~ 29%, cross section reduction 60 ~ 70%, impact toughness of 80 ~ 100J / cm ² Featured bolt By weight, it contains carbon 0.65-1.50%, silicon 2.0-4.0%, manganese 0.1-0.8%, chromium 0.1-0.8%, phosphorus 0.01% or less, sulfur 0.01% or less, nitrogen 0.005-0.01%, oxygen 0.005% or less Here, 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- One or two or more of the group consisting of 0.2% is selectively contained, and the steel workpiece composed of the remaining Fe and other impurities is heated at a temperature of Ac ₃ or more for 20 minutes or more to make an austenite single phase of 30 ° C./sec or more. After cooling to a temperature of Ms + 220 ° C. to Ms + 300 ° C. at a cooling rate, isothermal heat treatment for at least 60 minutes, followed by oil-cooling or air-cooling, the microstructure has a complex structure of ferrite + residual austenite, and the braky of the remaining austenite. Elongation attained in which the phase fraction of the residual austenite of the type is 10 to 30% And a high strength resistance composite having excellent impact toughness. 8. The method according to claim 7, wherein the carbon is contained in a range of 0.8% to 1.0%. 8. The method according to claim 7, wherein the silicon is contained in a range of 2.8 to 3.3%. 8. The method according to claim 7, wherein the phase fraction of the brachy type retained austenite is 15 to 25%. The method according to claim 7, wherein the steel workpiece is a bolt.