KR100345714B1 - Manufacturing method of bainite steel for high strength bolts with resistance ratio - Google Patents

Abstract

The present invention relates to a manufacturing method of bainite steel for high strength bolts having a resistive ratio, and its purpose is that in the microstructural aspect, the soft tissues preferentially begin to deform during tensile load and the breaking strength of the material is soft. By controlling the steel components and heat treatment conditions to be controlled in an appropriate range, it is to provide a method for producing a high-strength resistance ratio bolt steel having a tensile strength of 130kg / mm ² or more and a yield ratio of 0.7 or less.

The present invention for achieving the above object in the method for producing a steel for bolts, in weight percent, carbon 0.4-0.60%, silicon 2.0-4.0%, manganese 0.2-0.8%, chromium 0.25-0.8%, phosphorus 0.01% Or less, sulfur 0.01% or less, nitrogen 0.005-0.01%, oxygen 0.005% or less, including vanadium 0.05-0.2%, niobium 0.05-0.2%, nickel 0.3-2.0%, boron 0.001-0.003%, molybdenum 0.01 -Optionally contains one or two or more of the group consisting of -0.5%, 0.01-0.2% titanium, 0.01-0.5% tungsten, 0.01-0.2% copper, 0.01-0.5% cobalt, and the balance Fe and other unavoidable impurities The steel to be formed is heated at a temperature above Ac3 transformation point for 20 minutes or more to prepare austenite single phase, and then quenched to Ms + (160 ° C ± 50 ° C) range at a cooling rate of 70 ° C / sec or more, and isothermally heat treated for 20 minutes or more. Since the method of manufacturing a high-strength bolt bainite steel having a resistive ratio, characterized in that the oil-cooled or air-cooled To that point.

Description

Manufacturing method of bainite steel for high strength bolts with resistance ratio

The present invention relates to a method for manufacturing bolt steel used for construction and fastening of steel structures of sea-facing structures, and more particularly, to a method for producing bainite steel of high yield ratio high strength bolt using bainite structure. .

In order to reduce the weight of bolts, multifunction and high performance, it is necessary to increase the strength of materials. In the case of using high strength bolted steel, bolting in terms of steel structure does not require more skilled skills than welding, and considering the replacement of weak welds, it is possible to increase the stability of the steel structure due to the tightening force when tightening bolts. By reducing the number of bolt fastening has the advantage of reducing the steel consumption. Therefore, in recent years, the ratio of joining steel structure joints to bolted joints is increasing according to the size of the structure, the use environment, and the application characteristics.

On the other hand, the steel structure may cause the collapse of the structure due to earthquakes and strong winds, the steel material is required to have excellent shock resistance according to the use environment. As a method commonly used to prevent collapse of a structure by earthquakes and strong winds in steel structures, steel materials having different yield strengths are used in the case of steel materials used in structures (Japanese Patent Publication No. 96-42187), that is, a main structural member. Is composed of steel with high yield strength to support vertical loads, and the seismic members attached to the main structural members are composed of low yield strength steel, which is elastically deformed in the main member with high yield strength during earthquakes and high winds. The earthquake and the strong wind energy absorb shocks by the plastic (reverse) deformation of the seismic member, thereby improving the seismic resistance of the steel for steel structures.

In this sense, it is required to mix the high strength bolt steel and the high strength bolt steel with high seismic resistance with the same concept in the bolt connection part when manufacturing a steel structure having excellent seismic resistance. In order to have excellent seismic resistance, reduction of yield strength is necessary, and in order to impart seismic resistance at the bolted joint, it is inevitable to use a bolt steel having a resistance ratio in the bolted joint at an appropriate ratio. In order to maintain the advantages of bolted joints due to fastening, a steel for bolts having a high strength yield ratio is required.

Therefore, the advantages that are expected when high strength steel with low yield ratio and high yield strength bolt steel are developed is that the bolt connection part in the seismic design of steel structure is achieved while achieving the weight reduction, multifunctionality and high performance of the bolt for the efficient construction of the steel structure. By providing the seismic design, the seismic resistance of steel structures can be further improved.

Most of the existing microstructures used are quasi single phase tissues of tempered martensite, and the yield ratio is more than 0.8. By the way, it is because the distribution of soft tissue that can be preferentially deformed during tensile strain is impossible in the microstructure configuration to achieve a high-strength resistance ratio of the bolt material. In other words, if the tempering temperature for lowering the yield strength is increased, the yield strength is lowered, but at the same time, the tensile strength is also lowered, so the resistance yield ratio cannot be expected. Therefore, the production of high strength resistive bolt steel for the conventional tempered martensite structure is limited in terms of microstructure configuration characteristics.

Therefore, in order to achieve high strength and resistance ratio of the bolt, it is inevitable to construct a continuous structure of ferrite tissue having a low yield strength, and as a means to maximize the distribution of the ferrite structure during tensile deformation in the final microstructure. At the same time, it is desirable to achieve high strength.

There is no example of developing a steel for bolts having a resistance ratio, but Japanese Patent Laid-Open Publication No. 97-95734 is a technique for steel materials for rebar having excellent seismic resistance.

In the Japanese Patent Laid-Open Publication No. 97-95734, a method of manufacturing a steel shockproof concrete having a yield strength of 345 MPa or more and a yield ratio of 0.8 or less and a yield elongation of 1.4% or more and an impact toughness of 27 J / cm ² or more. Controlled cooling is made up of silicon 0.05-0.60%, manganese 0.60- 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, titanium 0.1% or less It is composed of, but not more than 130kg / mm ² high strength and resistance ratio (0.7 or less) was not achieved.

As a technique for producing high strength bainite + martensite composite structure low carbon alloy steel by heat treatment, Japanese Patent Application Laid-Open No. Hei 6-271975, Hei 7-173531, Japan "Iron and Steel" Vol. 82 (1996) No. 4th.

Japanese Patent Laid-Open No. 6-271975 relates to a method for manufacturing a composite tissue steel having excellent resistance to delayed fracture by hydrogen, wherein the weight percentage is 0.05-0.3% C, 0.1-2.5% Si, 0.1-3.0% Mn, 0.05-0.1 In steels containing at least one alloy element among Al, Cu, Ni, Mo, Cr, Nb, V, Ti, and B, the microstructure is a martensite single phase, bainite single phase, or bainite + martensite composite structure. The microstructure is characterized in that the residual austenite is present in the volume fraction of 1-30% in order to secure the resistance to delayed destruction by hydrogen. However, in Japanese Patent Application Laid-Open No. 6-271975, the yield ratio was not achieved while securing a tensile strength of 130 kg / mm ² or more.

Patent Publication No. Hei 7-173531 has a chemical composition of 0.05-0.3% C, 0.05-2.0% Si, 0.3-5.0% Mn, 1.0-3.0% Cr, 0.01-0.5% Nb, 0.01-0.06% Al by weight. The present invention relates to a method for manufacturing bainite + martensite abnormal composite tissue steel by continuously cooling the steel having hot forming at a critical cooling rate at which the cornerstone ferrite does not precipitate, but having a critical delay fracture strength of 130kg / mm ² or more. It could not be achieved.

Japanese "Iron and Steel" Vol. 82 (1996) No.4 is based on the conventional tempered martensitic structure, based on the alloy composition 0.49C-0.31% Mn-1.02% Cr-0.68% Mo-0.034% Nb-0.32% V-0.009% P-0.004% S Its grain boundary delay strength is 130kg / mm ² and it is made low P, low S, low Mn to reduce grain boundary segregation of impurities, Ni, Cr, Mo, V is added to prevent grain boundary precipitation of carbide, In order to refine the grains, V, Nb, and Ti are added to heat treatment at a low tempering temperature. However, the Japanese Iron and Steel Vol. 82 (1996) No. In 4, it was not possible to achieve the problems and the resistance ratio, which are difficult to secure the tensile strength of 130kg / mm ² or more.

Accordingly, the present invention has been made to solve the above problems, its purpose is that in the microstructural aspect, the initial tendency of soft tissue deformation during tensile load and the breaking strength of the material is controlled so as to control the hard tissue and By controlling the heat treatment conditions to an appropriate range, it is to provide a method for producing a high strength resistance ratio bolt steel having a tensile strength of 130kg / mm ² or more and a yield ratio of 0.7 or less.

Figure 1 (a) is an invention example, (b) is a microstructure photograph using the SEM of the comparative example

The present inventors conducted a multi-angle study on the control method of the microstructure having high strength resistance ratio, and found that 2.0% -4.0% silicon, vanadium, chromium, manganese, nickel, tungsten molybdenum, copper, boron, titanium, etc. in medium carbon steels were selected. When heat-treated at Ac3 transformation point after heat treatment and rapidly cooled to Marsite transformation point (Ms) in the range of Ms + (160 ℃ ± 50 ℃), and then isothermally heat-treated to produce bainite tissue, We found that we could manufacture the visible bolted steel.

Starting from the above point of view, the present invention relates to a method for producing steel for bolts in terms of weight%, carbon 0.4-0.60%, silicon 2.0-4.0%, manganese 0.2-0.8%, chromium 0.25-0.8%, phosphorus 0.01% or less, sulfur 0.01% or less, nitrogen 0.005-0.01%, oxygen 0.005% or less, including vanadium 0.05-0.2%, niobium 0.05-0.2%, nickel 0.3-2.0%, boron 0.001-0.003%, Molybdenum 0.01-0.5%, Titanium 0.01-0.2%, Tungsten 0.01-0.5%, Copper 0.01-0.2%, Cobalt 0.01-0.5%, optionally containing one or two or more, the balance Fe and other unavoidable After heating the steel composed of impurity for more than 20 minutes at the temperature of Ac3 transformation point or more to prepare austenite single phase, and then quenched to Ms + (160 ℃ ± 50 ℃) range with cooling rate of 70 ℃ / sec or more, and then isothermally heat treated for 20 minutes or more. And afterwards, production of high-strength bolt bainite steel having a resistive ratio, characterized in that it is oil-cooled or air-cooled. Relate to.

Hereinafter, the reason for limitation of the said component and a component range is demonstrated.

The reason for limiting the content of carbon (C) to 0.40-0.60% is that if the content is less than 0.40%, it is difficult to secure sufficient tensile strength as a high strength bolt steel after heat treatment for bainite production, and exceeds 0.60%. This is because it affects the difficulty of securing toughness after heat treatment, decarburization of the furnace, permanent deformation during bolting, fatigue characteristics, carbonitride distribution, bainite structure, and bainite transformation time.

The content of silicon (Si) is limited to 2.0-4.0%. If the content is less than 2.0%, the solid solution strengthening effect of ferrite in bainite structure is insufficient, which makes it difficult to secure the strength, and also affects delayed fracture resistance, surface corrosion characteristics, impact toughness, and permanent deformation during bolting. In addition, due to the difficulty in proper distribution of the surface ferrite decarburization layer in the wire heating furnace for controlling the decarburization of the wire, the decarburization is intensified, and it is difficult to control the surface scale characteristics due to the increase in hardenability during wire cooling. In addition, in the case of exceeding 4.0%, the above-mentioned effect is not preferable because it saturates and affects bainite structure, impact toughness, and fatigue properties to obtain hardenability and resistance ratio. This is because the quality characteristics in the final product are deteriorated due to the inhomogeneity of microstructures due to silicon segregation during the manufacture of billets, and because the thickness of the surface ferrite layer increases during heat treatment, it is difficult to control homogeneous surface decarburization. to be.

The more preferable component range of the silicon is 2.8-3.3% isothermal heat treatment time and residual austenite fraction for producing bainite structure, bainite structure composition for obtaining bainite high strength, resistivity ratio, delayed fracture resistance (diffusion property) This is because it can be effectively improved in consideration of hydrogen content, scour control of grain boundary precipitates, surface decarburization, stress relaxation or permanent deformation resistance after bolting, and dynamic and static fatigue characteristics.

The content of manganese (Mn) is limited to 0.2-0.8%. The reason for this is that manganese forms a solid solution to form a solid solution in the matrix structure, and is a very useful element for high-strength bolt characteristics, but when it is added more than 0.8%, the tissue heterogeneity due to manganese segregation is higher than that of solid solution. Has a more harmful effect on properties. In addition, when less than 0.2% of manganese is added, the formation of segregation zone due to manganese segregation is hardly achieved, but stress relaxation improvement effect due to solid solution strengthening is not expected. In other words, if the content of manganese is less than 0.2%, the improvement of hardenability and permanent deformation resistance is insufficient due to insufficient solidification effect, and if it is more than 0.8%, tissue anisotropy is increased due to increased local quenchability and segregation zone due to manganese segregation during casting. Deepening, ie, tissue heterogeneity, causes the bolt properties to deteriorate. Therefore, limiting the content of manganese to 0.2-0.8% is a range in consideration of the strength of the base material, the hardenability during heat treatment, the stress relaxation, and the harmful effects of segregation zone formation.

The content of chromium (Cr) is limited to 0.25-0.8%. The reason is that less than 0.25% is difficult to form the surface ferrite layer for surface decarburization control during heat treatment of high silicon-added steel, so it is hardly expected to suppress decarburization and hardly improve quenchability. This is because it is not preferable because the transformation time of bainite is long, and it is difficult to generate a surface titration ferrite layer when charging the wire rod for controlling the wire decarburization layer, which affects homogeneous decarburization control.

The vanadium (V) or niobium (Nb) is an element that improves delayed fracture resistance and stress relaxation resistance, and its content is limited to 0.05-0.2%. If the content of these components is less than 0.05%, the distribution of vanadium or niobium-based precipitates in the base material becomes less, 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. This is because it is difficult to expect the improvement effect on the stress relaxation resistance is not sufficient, and it is difficult to expect the austenite grain refinement, which affects the structure of bainite structure. In addition, when these contents exceed 0.2%, the improvement effect on the delayed fracture resistance and the stress relaxation resistance by the precipitates is saturated, and the amount of coarse alloy carbide which is not dissolved in the base metal during the austenitic heat treatment increases, thus acting as a non-metallic inclusion. This is because the fatigue property is lowered.

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

The content of nitrogen (N) is limited to 0.005-0.03%. If the content is less than 0.005%, it is difficult to form vanadium and niobium-based nitrides acting as non-diffusion hydrogen trap sites. If the content exceeds 0.03%, the effect is saturated.

The content of phosphorus (P) and sulfur (S) is limited to 0.01% or less. The phosphorus segregates at the grain boundaries and lowers the toughness, so the upper limit thereof is limited to 0.01%. The sulfur is segregated into low-melting elements, which degrades the toughness and forms an emulsion, which has a detrimental effect on delayed fracture resistance and stress relaxation characteristics. It is desirable to limit the upper limit to 0.01% since it is insane.

The nickel (Ni) is an element that forms a nickel enriched layer on the surface during heat treatment to suppress permeation of external hydrogen and thereby improves delayed fracture resistance. If the content is 0.3-2.0%, it is difficult to expect improvement effect of delayed fracture resistance due to incomplete surface thickening layer formation at less than 0.3%, and also long heat treatment time during graphitization treatment for decarburization control, toughness, and diversification of manufacturing process. This is because there is no improvement in cold formability during cold bolt processing, and if it exceeds 2.0%, the effect is saturated and the temper embrittlement occurs during tempering due to an increase in the amount of retained austenite during quenching. to be.

The boron (boron, B) in the present invention is limited to the content of boron as a grain boundary strengthening element for improving the hardenability and the fracture resistance to 0.0010-0.003%. If the content is less than 0.0010%, the effect of improving grain boundary strength due to grain boundary strengthening due to grain boundary segregation of boron atoms during heat treatment is insufficient. This is because the addition effect is saturated and rather the precipitation of boron nitride in the grain boundary leads to a decrease in grain boundary strength.

The content of the molybdenum (Mo) and tungsten (W) is limited to 0.01-0.5%. If the content is less than 0.01%, it is difficult to obtain the effect of improving stress relaxation by inhibiting the growth of cementite when cementite transitions from the grain carbide to tempering, and finely distributes the molybdenum-based precipitates at high temperature during tempering. This is because it is difficult to secure a stable organization. In addition, when these contents exceed 0.5%, the effect is saturated, and the hardenability is increased, so that low-temperature structure (martensite + bainite) is easily produced during wire rod manufacturing, and heat treatment time is increased during graphitization treatment to improve cold formability. This is because there is a disadvantage.

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

The content of titanium is limited to 0.01-0.2%. If the content is less than 0.01%, the effect of miniaturizing austenite crystal grains is insufficient, and the precipitation distribution of titanium-based carbon and nitride, which is effective for delayed fracture resistance, is insufficient. Therefore, the improvement effect is difficult to be expected. Is saturated and coarse titanium-based nitride is harmful to fatigue properties.

The content of cobalt is limited to 0.01-0.5%. If the content is less than 0.01%, the softening effect is not sufficient in spheroidization or graphitization heat treatment, which is a material soft-nitrification heat treatment for cold forging, and it has no effect on the grain boundary diffusive hydrogen concentration. It is undesirable because it is saturated and the soft nitriding rate is increased greatly during softening heat treatment, resulting in partial microstructure heterogeneity during heat treatment.

Hereinafter, a method of manufacturing bainite steel for bolts using steel having the chemical composition as described above will be described in detail.

In the present invention, the austenite temperature and time is 20 minutes or more at the Ac3 transformation point below 1050 ° C., but below the Ac3 transformation point, the austenite single phase and sufficient austenitization are not achieved as an abnormal region where the ferrite and austenite phase coexist. This is because it can lead to microstructure heterogeneity in the production of bainite tissue to show the effect. Above 1050 ° C, surface decarburization and coarse grain formation of austenite are caused when the material is heated, resulting in quality characteristics and coarse cementite in the final product. This is because the constructed bainite structure is detrimental to impact toughness, tensile and yield strength, stress relaxation, and fatigue characteristics. In addition, when the heating time is 20 minutes or less, it is difficult to secure sufficient bainite structure because sufficient austenitization is not achieved.

On the other hand, after isothermal heat treatment conditions for the manufacture of bainite structure after heating is limited to the range of Ms + (160 ℃ ± 50 ℃) directly above the martensite transformation temperature (Ms) as follows.

That is, below Ms + 110 ° C, the cementite structure has a thin thickness of cementite, and the ferrite distribution layer in the base material is very finely distributed with cementite. This is because it is difficult to lower the strength, and if it exceeds Ms + 210 ° C, it is impossible to increase the tensile strength due to the cornerstone ferrite transformation and the pearlite transformation, and the yield strength increases and the impact toughness decreases drastically.

The more desirable bainite transformation temperature range with high strength and resistivity ratio is Ms + (160 ± 20 ° C), which means yield ratio, shock resistance, bolt fastening, corrosion resistance, stress relaxation during bolting, impact toughness, elongation, tensile It is the range which considered intensity | strength.

Hereinafter, the present invention will be described in detail through examples.

Example

Steels having a component composition as shown in Table 1 were prepared, and the inventive material (1-8) satisfies the component range of the present invention, and the comparative material (1-4) is those outside the component range of the present invention. The prepared steels were cast into 50 kg ingot, homogenized heat treated at 1250 ° C. for 48 hours, and hot rolled to 13 mm in thickness. At this time, the finishing temperature was 950 ℃ or more and hot-rolled after hot rolling, the rolling ratio was 80% or more.

C Si Mn Cr V Ni Mo Ti W B P S N ₂ Invention 1 0.45 3.03 0.29 0.58 0.05 - - - - - 0.005 0.004 0.008 Invention 2 0.40 3.42 0.31 0.79 0.2 - - 0.01 - 0.0013 0.006 0.005 0.014 Invention 3 0.60 2.99 0.32 0.33 0.05 0.54 - - 0.02 - 0.007 0.009 0.007 Invention 4 0.45 2.0 0.77 0.51 0.11 - 0.2 0.03 - - 0.006 0.008 0.009 Invention 5 0.44 3.96 0.23 0.27 0.06 - - - 0.2 0.0015 0.008 0.008 0.008 Invention 6 0.53 3.01 0.35 0.55 - - 0.05 0.05 0.07 0.0010 0.004 0.009 0.004 Invention 7 0.58 2.56 0.80 0.29 - 1.10 0.13 0.10 - - 0.005 0.006 0.005 Invention Material 8 0.44 3.1 0.34 0.55 0.07 - - - - - 0.007 0.006 0.008 Comparative Material 1 0.35 0.19 0.67 0.95 tr. 0.03 0.17 - - - 0.019 0.015 0.004 Comparative Material 2 0.31 0.20 0.62 0.95 tr. 0.04 0.05 - - - 0.017 0.010 0.005 Comparative Material 3 0.34 0.22 0.36 1.26 0.019 0.05 0.40 - - - 0.011 0.012 0.015 Comparative Material 4 0.20 0.20 0.80 0.72 - - 0.04 - - 0.0015 0.009 0.004 0.005

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

At this time, the heat treatment conditions were subjected to a heat treatment test under the heating and isothermal heat treatment conditions shown in Tables 2 and 3 below. In addition, Ms transformation temperature was measured using a thermal analyzer (dilatometry), the results are shown in Table 2 and Table 3.

Steel grade used Heating temperature (℃) Heating time (min) Isothermal heating (℃) (Ms + X) Tempering temperature (℃) Isothermal holding time (min) Tempering time (min) Ms transformation temperature (℃) Inventive Example 1 Invention 1 950 30 X = 110 30 290 Inventive Example 2 Invention 1 950 30 X = 160 30 290 Inventive Example 3 Invention 1 950 30 X = 210 30 290 Inventive Example 4 Invention 1 1000 20 X = 160 30 287 Inventive Example 5 Invention 1 1050 20 X = 160 30 285 Comparative Example 1 Invention Material 8 950 30 X = 60 30 290 Comparative Example 2 Invention Material 8 950 30 X = 80 30 290 Comparative Example 3 Invention Material 8 950 30 X = 240 30 290 Comparative Example 4 Invention Material 8 950 30 X = 280 30 290 Comparative Example 5 Invention Material 8 950 30 Tempering temperature = 300 60 - Comparative Example 6 Invention Material 8 950 30 Tempering temperature = 400 60 Comparative Example 7 Invention Material 8 950 30 Tempering temperature = 500 60 Comparative Example 8 Invention Material 8 950 30 Tempering temperature = 600 60

Steel grade used Heating temperature (℃) Heating time (min) Isothermal heating (℃) (Ms + X) Tempering temperature (℃) Isothermal holding time (min) Tempering time (min) Ms transformation temperature (℃) Inventive Example 6 Invention 2 1000 40 X = 160 30 307 Inventive Example 7 Invention 3 1000 40 X = 160 30 247 Inventive Example 8 Invention 4 1030 40 X = 160 30 278 Inventive Example 9 Invention 5 1030 40 X = 160 30 304 Inventive Example 10 Invention 6 1030 40 X = 160 30 268 Inventive Example 11 Invention 7 1050 40 X = 160 30 223 Comparative Example 9 Comparative Material 1 900 30 Tempering temperature = 450 60 - Comparative Example 10 Comparative Material 2 900 30 Tempering temperature = 450 60 Comparative Example 11 Comparative Material 3 950 30 Tempering temperature = 450 60 Comparative Example 12 Comparative Material 4 900 30 Tempering temperature = 450 60

Inventive Example 1-5 in Table 2 is an austenitic single-phase heating temperature range of the same alloy component system (Inventive material 1) is heated at 1050 ℃ or less for 40 minutes at the Ac3 transformation point, isothermal heat treatment temperature range for bainite transformation Ms + (160 ± 50) ℃ to quenched at a cooling rate of 70 ℃ / sec or more and prepared by heat treatment for 40 minutes. Where Ms is the martensite transformation start temperature.

Comparative Example 1-4 in Table 2 shows that the austenitic single-phase induction heating temperature of the same alloy system (Inventive Material 8) is heated at 950 ° C. for 30 minutes above the Ac 3 transformation point, and isothermal heating temperature of Ms + 110 or less and Ms + 210 or more. It was prepared by heating at least for 40 minutes.

In addition, in Table 2, Comparative Example 5-8 was prepared with a conventional tempered martensite structure having a ferrite phase percentage of 0% in the same alloy component system (Inventive Material 8).

In Table 3, Inventive Example 6-11 was heated for 40 minutes in the range below 1050 ° C at the Ac3 transformation point, which is the austenitic single-phase heating temperature range for each alloy component of Inventive Materials 2-7, and then to Ms + 160 for bainite transformation. The liquid was quenched at a cooling rate of 70 ° C./sec or more, and then oil-cooled after 40 minutes of isothermal holding time.

On the other hand, Comparative Example 9-12 was heated in the 900 ~ 950 ℃ range of austenite single phase region, oil cooled and tempered at 450 ℃.

In order to investigate the tensile and impact properties of the materials manufactured as described above, the tensile test piece was used in the KS standard (KS B 0801) No. 4 test piece and the tensile test at a cross head speed of 5mm / min Tested. 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).

Tensile properties and impact toughness of the inventive examples and comparative examples prepared as described above were measured, and the results are shown in Tables 4 and 5 below. On the other hand, the microstructured difference between the invention example and the comparative example is shown in Figure 1, Inventive Example 2 in Figure 1 (a) is a bainite structure showing a resistance ratio, Comparative Example 1 (b) is a typical tempered martensite Organization.

Tensile Strength (kg / mm ² ) Yield strength (kg / mm ² ) Elongation (%) Cross section reduction rate (%) Impact Toughness (J / cm ² ) Yield fee Inventive Example 1 130 80 15 40 90 0.62 Inventive Example 2 155 60 15 30 50 0.39 Inventive Example 3 150 50 15 35 55 0.33 Inventive Example 4 150 58 15 35 52 0.39 Inventive Example 5 147 55 15 30 48 0.37 Comparative Example 1 150 115 14 60 135 0.77 Comparative Example 2 145 100 15 63 130 0.69 Comparative Example 3 125 99 13 50 50 0.79 Comparative Example 4 116 87 13 52 45 0.75 Comparative Example 5 225 195 6 25 20 0.87 Comparative Example 6 220 190 8 30 25 0.86 Comparative Example 7 180 165 10 35 40 0.92 Comparative Example 8 145 130 11 40 30 0.90

Tensile Strength (kg / mm ² ) Yield strength (kg / mm ² ) Elongation (%) Cross section reduction rate (%) Impact Toughness (J / cm ² ) Yield fee Inventive Example 6 162 75 15 40 60 0.46 Inventive Example 7 174 80 15 42 55 0.46 Inventive Example 8 164 70 16 45 75 0.43 Inventive Example 9 165 71 15 40 63 0.43 Inventive Example 10 159 75 16 43 70 0.47 Inventive Example 11 178 90 15 45 80 0.50 Comparative Example 9 147 135 15 57 30 0.92 Comparative Example 10 147 129 16 58 20 0.88 Comparative Example 11 148 139 15 57 40 0.94 Comparative Example 12 110 95 15 60 50 0.86

As shown in Table 4 and Table 5, the invention examples satisfying the conditions of the present invention have a yield ratio of 0.33-0.620 while the tensile strength is 130-178kg / mm ² level, Comparative Examples 0.69-0.94 However, it can be seen that the steels according to the present invention using the bainite structure at the tensile strength of 110-225kg / mm ² can significantly improve the yield ratio while having the same level of strength as the existing steel. Here, the bainite transformation temperature for showing the effect of the present invention was found to be the most preferable Ms + (160 ℃ ± 20 ℃) range.

On the other hand, the effect of the present invention as shown in Figure 1 has a continuous distribution form of the ferrite in the base material on the bainite structure configuration, when the tensile strain, the soft structure of the ferrite preferentially deformed tensile strength is more than 130kg / mm ² In addition, it is possible to secure a resistance yield ratio indicating a low yield strength.

As described above, the present invention proposes an alloy composition system and heat treatment conditions of bainite steel that can increase the strength of the tensile strength while having the strength characteristics of the resistance ratio, thereby achieving a higher resistance ratio of the bolt and at the same time securing the high strength of the bolt. As a result, it is possible to provide bainite steel for high strength and resistive bolts.

Claims (4)

In the method of manufacturing the steel for bolts, By weight%, contains carbon 0.4-0.60%, silicon 2.0-4.0%, manganese 0.2-0.8%, chromium 0.25-0.8%, phosphorus 0.01% or less, sulfur 0.01% or less, nitrogen 0.005-0.01%, oxygen 0.005% or less In addition, vanadium 0.05-0.2%, niobium 0.05-0.2%, nickel 0.3-2.0%, boron 0.001-0.003%, molybdenum 0.01-0.5%, titanium 0.01-0.2%, tungsten 0.01-0.5%, copper 0.01- Austenitic single phase is prepared by optionally heating one or two or more of the group consisting of 0.2% and 0.01-0.5% of cobalt, and heating the steel composed of residual Fe and other unavoidable impurities at a temperature of Ac3 transformation point for at least 20 minutes. After the quenching to the Ms + (160 ℃ ± 50 ℃) range at a cooling rate of 70 ℃ / sec or more isothermal heat treatment for 20 minutes or more, and then the oil or air-cooled high strength bolt bainite steel Manufacturing method The method of claim 1, Method for producing high strength bolt bainite steel having a resistive ratio, characterized in that the silicon is contained in the range of 2.8-3.3% The method of claim 1, Method for producing high-strength bolt bainite steel having a resistive ratio, characterized in that heat treatment for more than 20 minutes in the range below 1050 ℃ at Ac3 transformation point or more. The method of claim 1, The isothermal heating temperature is Ms + (160 ℃ ± 20 ℃) method for producing a high-strength bolt bainite steel having a resistance ratio, characterized in that limited to