KR20000037998A - Method for producing composite structured steel for high-strength bolt having low breakdown ratio - Google Patents

Abstract

PURPOSE: A method for producing compound structure steel is provided to produce a high-strength bolt having low breakdown ratio and over 130kg per squaremeter of tension strength by controlling steel composition and thermal treatment condition in a proper range. CONSTITUTION: In producing steel for a bolt, 0.4-0.6wt% of carbon, 2.0-4.0wt% of silicon, 0.2-0.8wt% of manganese, 0.25-0.8wt% of chrome, less than 0.01wt% of phosphor, less than 0.01wt% of sulphur, 0.005-0.01wt% of nitrogen, and less than 0.005wt% of oxygen are contained. More than one or two of element are selected from a group composed of 0.05-0.2wt% of vanadium, 0.05-0.2wt% of niobium, 0.3-2.0wt% of nickel, 0.001-0.003wt% of boron, 0.01-0.5wt% of molybdenum, 0.01-2.0wt% of titanium, 0.01-0.5wt% of tungsten, 0.01-0.2wt% of copper, and 0.01-0.5wt% of cobalt. And residual iron and impurities are contained. The steel is heated within Ac3-(Ac3-Ac1)/1.3 and Ac3-(Ac3-Ac1)/5.5 for more than 20 minutes to be produced in composite structure of 5-25% of ferrite composition ratio. The steel is cooled rapidly within the temperature ratio of Ms+(120°C and 180°C) at more than 70°C/sec of cooling speed. Isothermal treatment is performed for more than 20 minutes. Fluid cooling or air cooling is performed thereafter. Thereby, compound structure steel for a high-strength bolt is produced.

Description

METHOD FOR MANUFACTURING COMPOSITE TEXTILE STEEL FOR HIGH STRENGTH BOLT

The present invention relates to a method of manufacturing a bolt steel for use in fastening steel constructions of construction and shore structures, and more particularly, to a method of manufacturing a composite structure steel for high bolt high strength bolt using ferrite + bainite structure .

In order to make member fastening and bolt lightweight for efficient construction of steel structure, and to make multifunction and high performance, it is required to increase the strength of material. When using a high strength bolt steel, the bolt fastening on the side of the steel structure does not require a skillful technique as compared with the weld joint, and considering the substitution of a weak welded part, stability of the steel structure due to strengthening of the fastening force can be improved There is an advantage that the amount of steel used can be reduced by reducing the number of bolts fastened. Therefore, according to the recent structure size, use environment, and application characteristics, the ratio of steel-joint joining method is increasing from welding to bolt joining method.

On the other hand, the steel structure may cause the collapse of the structure due to the earthquake and the strong wind, and a steel material excellent in vibration resistance is required according to the use environment. As a method commonly used to prevent collapse of structures due to earthquakes and strong winds in steel structures, steels having different yield strengths in the case of steels used in structures are used (Japanese Patent Application Laid-Open No. 96-42187) The member is made of a steel having a high yield strength to support a vertical load and the vibration-proofing member attached to the main structural member is made of a steel having a low yield strength. Thus, in a large- And the energy of the strong earthquake and the strong wind absorbs the impact by the plastic deformation of the earthquake-proof member, thereby improving the seismic resistance of the steel material for steel structure.

In this sense, when manufacturing a steel structure having excellent vibration resistance, it is required to mix bolt and high strength bolt steels excellent in vibration resistance with the same concept in the bolt fastening part. In order to obtain excellent vibration resistance, it is necessary to reduce the yield strength. In addition, in order to provide vibration resistance at the bolt fastening portion, it is inevitable to use bolt steel having a low resistance ratio at a proper ratio in the bolt fastening portion. Bolt steels having a high strength and low resistance ratio are required to maintain the advantages of the bolted connections upon fastening.

Therefore, the advantages expected when a high-strength resistant low-bolt molten steel having high strength and low yield ratio is developed include member fastening, weight reduction, multi-function and high performance for efficient construction of steel structure, The seismic resistance of the steel structure can be further improved by providing an earthquake-resistant design.

Most of the existing microstructures are quartz single phase structures of tempered martensite, and the yield ratio is 0.8 or more. However, the major impediment to achieving the high strength resistance reduction of the bolt material is that the distribution of soft tissues, which can be preferentially deformed during tensile deformation, is impossible due to the microstructure. That is, when 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 that the low resistance can not be expected. Therefore, the production of the high-strength resistant low-bolt molten steel with the conventional tempered martensite structure has limitations in the microstructure configuration characteristics.

Therefore, in order to achieve high strength and low resistance of the bolt, it is inevitable to construct a ferrite structure having a low yield strength as a continuous structure. As a means for this purpose, the ferrite structure distribution in the final microstructure is maximized While achieving high strength at the same time.

There is no development example of a bolt steel having a low resistance, but Japanese Patent Laid-Open Publication No. 97-95734 is known as a technique for a steel material for reinforcing steel excellent in vibration resistance.

Japanese Patent Laid-Open Publication No. 97-95734 discloses a method of producing a steel for reinforced concrete having an excellent yield strength of not less than 345 MPa and a yield ratio of not more than 0.8 and a yield strength of not less than 1.4% and an impact toughness of not less than 27 J / cm ² , Controlled cooling is made up of 0.05-0.60% silicon, 0.60-2.0% manganese, 0.005-0.080% aluminum, 0.008% boron, 0.3% copper, 0.3% nickel, 0.1% molybdenum and 0.1% However, it was not able to achieve a high strength of 130 kg / mm ² or more and a low resistance (0.7 or less).

As a manufacturing technique of a high-strength bainite + martensite composite low-carbon alloy steel by heat treatment, Japanese Patent Application Laid-Open Nos. 6-271975 and 7-173531, "Iron and Steel" Vol. 82 (1996). 4, and so on.

Japanese Patent Application Laid-Open No. Hei 6-271975 discloses a method for manufacturing a composite structure steel excellent in delayed fracture resistance due to hydrogen, which comprises 0.05-0.3% C, 0.1-2.5% Si, 0.1-3.0% Mn, The microstructure is a martensite single phase, a bainite single phase, or a bainite + martensite composite structure in a steel containing at least one of Al, Cu, Ni, Mo, Cr, Nb, V, Characterized in that the retained austenite has a volume fraction of 1-30% in order to ensure delayed fracture resistance of the microstructure due to hydrogen. However, in JP-A-6-271975, a tensile strength of 130 kg / mm < ² >

The above-mentioned Japanese Patent Application Laid-Open No. 7-173531 discloses 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 and 0.01-0.06% The present invention relates to a method of producing a bainite + martensitic composite steel having a critical retardation breaking strength of at least 130 kg / mm ² by continuous cooling at a temperature not lower than a critical cooling rate at which a precious- I did not achieve it.

Japan "Iron and Steel" Vol. 82 (1996) No. 4 is based on the conventional tempered martensite structure and contains 0.49C-0.31% Mn-1.02% Cr-0.68% Mo-0.034% Nb-0.32% V-0.009% P-0.004% S configuration and strength of the grain boundaries delayed fracture is added to the low P, low S, low Mn screen, for grain boundary precipitation prevention of carbide Ni, Cr, Mo, and V for grain boundary segregation, the reduction of impurities to 130kg / mm ² class in, and V, Nb and Ti are added for grain refinement and heat treatment is performed at a low tempering temperature. However, the Japanese Iron and Steel Vol. 82 (1996). 4, it was not possible to achieve a tensile strength of 130 kg / mm ² or more and a low resistance.

The present invention has been made in order to solve the above problems, and it is an object of the present invention to provide a method of manufacturing a steel structure, in which a soft structure is firstly deformed at the beginning of a tensile load, And a method of manufacturing a steel for bolts having a tensile strength of 130 kg / mm < ^{2 >} or more and a low resistance ratio by controlling the conditions to an appropriate range.

Fig. 1 (a) is a drawing, Fig. 1 (b) is a microstructure photograph

As a result of various studies on the microstructure control method having a high strength and low resistance ratio, the present inventors have found that the use of silicon, vanadium, chromium, manganese, nickel, tungsten molybdenum, copper, boron, (Ferrite + austenite), and isothermally heat-treated after being quenched in the range of martensitic transformation point (Ms), Ms + (150 ° C ± 30 ° C) It has been found that when the austenite in the structure is transformed into the bainite structure, the improvement effect on the delayed fracture resistance is remarkable, and the bolt steel having high strength and low resistance can be produced.

The present invention, starting from the above-mentioned viewpoints, is a method for producing a steel for bolts, which comprises 0.4-0.60% carbon, 2.0-4.0% silicon, 0.2-0.8% manganese, 0.25-0.8% chromium, 0.01 to 0.01% sulfur, 0.01% or less of sulfur, 0.005 to 0.01% of nitrogen and 0.005% or less of oxygen, wherein 0.05 to 0.2% of vanadium, 0.05 to 0.2% of niobium, 0.3 to 2.0% of nickel, 0.001 to 0.003% And the balance Fe and other inevitable impurities are contained in an amount of 0.01-0.5% of molybdenum, 0.01-0.2% of titanium, 0.01-0.5% of tungsten, 0.01-0.2% of copper and 0.01-0.5% of cobalt. The steel made from the impurities was heated for 20 minutes or more within the range of Ac3- (Ac3-Ac1) /1.3 to Ac3- (Ac3-Ac1) /5.5 to prepare a composite structure having a ferrite phase fraction of 5-25% (150 占 폚 占 30 占 폚) at a cooling rate of 70 占 폚 / sec or more, followed by isothermal heat treatment for 20 minutes or more, and thereafter subjected to oil cooling or air cooling. It is a method for producing Dual Phase Steel for high strength bolts.

Hereinafter, reasons for limiting the above-described components and ranges of components will be described.

The reason why the content of carbon (C) is limited to 0.40-0.60% is that when the content is less than 0.40%, it is difficult to secure a sufficient tensile strength as a high strength bolt steel after heat treatment for producing bainite, and more than 0.60% The reason for this is that it is difficult to secure toughness after heat treatment and that the carbon concentration of austenite due to ferrite formation during the anomaly treatment is too high to secure a bainite structure effective for high elongation. Carbonitride distribution, bainite structure, bainite transformation time and so on.

The content of silicon (Si) is limited to 2.0-4.0%. When the content is less than 2.0%, the ferrite hardening effect of the ferrite in the bainite structure is insufficient and it is difficult to secure the strength. In addition, it is difficult to secure the strength of the ferrite in the bainite structure and also the retardation fracture resistance, surface corrosion resistance, impact toughness, And it is difficult to control the surface scale characteristics due to the increase of the ingot property upon cooling the wire rod because of the difficulty in proper distribution of the surface ferrite decarburized layer in the wire rod heating furnace for decarburization control of wire rods to be. On the other hand, if it exceeds 4.0%, the above-mentioned effect is saturated and it is not preferable because it affects the bainite structure, impact toughness, fatigue characteristics, ) Or billets due to the segregation of the microstructure due to silicon segregation and deteriorate the quality characteristics in the final product. Further, since the thickness of the surface ferrite layer increases during the heat treatment, it is difficult to control the decarburization homogeneous surface to be.

The more preferable composition range of the silicon is 2.8-3.3%, and the isothermal heat treatment time and the austenite fraction for producing bainite structure, the bainite structure for obtaining the high strength of bainite, the bainite structure, the delayed fracture resistance This is because the stress relaxation or permanent deformation resistance after the bolt fastening or the dynamic and static fatigue characteristics can be improved very effectively.

The content of manganese (Mn) is limited to 0.2-0.8%. The reason for this is that manganese is a very useful element for high tensile strength of bolts due to the formation of substitutional solid solution in matrix matrix. However, when added more than 0.8%, the structure heterogeneity due to manganese It has a more detrimental effect on traits. In addition, when manganese is added in an amount of less than 0.2%, there is almost no formation of a segregation zone due to manganese grains, but it is difficult to expect an effect of improving stress relaxation due to solid solution strengthening. When the content of manganese exceeds 0.8%, it is reduced to manganese segregation during casting, but local anisotropy is increased due to increase of local incombustibility and formation of segregation zone. Therefore, the content of manganese is limited to 0.2-0.8% considering the strength of the base material, the fineness at the time of heat treatment, the completion of stress, and the harmful effects of segregation.

The content of chromium (Cr) is limited to 0.25-0.8%. The reason for this is that when the content is less than 0.25%, it is difficult to form a surface ferrite layer for surface decarburization control in the heat treatment of high silicon added steel, This is because it is not preferable because the time required for transformation of bainite is prolonged. It is difficult to generate a surface titled ferrite layer when the wire rod is heated for wire rod decarburization layer control, which affects the homogeneous decarburization control.

The vanadium (V) or niobium (Nb) limits the content of delayed fracture resistance and stress as improving elements to 0.05-0.2%. When the content of these components is less than 0.05%, the distribution of vanadium or niobium-based precipitates in the base material is decreased, and the role as a non-proliferative hydrogen trap site is insufficient, so that it is difficult to expect the effect of improving the delayed fracture resistance. This is because it is difficult to expect an improvement effect on stress relaxation resistance and it is difficult to expect finer austenite grains, which affects the bainite structure. When the content exceeds 0.2%, the improvement effect on the delayed fracture resistance and the stress relaxation resistance due to the precipitates is saturated, and the amount of coarse alloy carbide that is not dissolved in the base material during the austenite heat treatment is increased, The fatigue characteristics are deteriorated.

The content of oxygen (O) is limited to 0.0015% or less. If the content exceeds 0.0015%, coarse oxide based nonmetal inclusions are easily formed and the fatigue life is lowered.

The content of nitrogen (N) is limited to 0.005-0.03%. When the content is less than 0.005%, it is difficult to form vanadium and niobium nitride acting as non-proliferative 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 is segregated in the crystal grain boundaries to lower the toughness so that the upper limit is limited to 0.01%. The sulfur is segregated with low-melting point elements to lower the toughness and form an emulsion, thereby detrimentally affecting the delayed fracture resistance and stress relaxation characteristics It is preferable to limit the upper limit to 0.01%.

The nickel (Ni) is an element which forms a nickel-enriched layer on the surface during heat treatment to inhibit permeation of external hydrogen to improve delayed fracture resistance. When the content is set to 0.3 to 2.0%, it is difficult to expect the effect of improving the delayed fracture resistance due to incomplete formation of the surface-concentrated layer at less than 0.3%. In addition, the graphite treatment for decarburization control, toughness and diversification of the manufacturing process has a long heat treatment time And the effect of improving the cold formability during cold bolting is not exhibited. If it exceeds 2.0%, the effect becomes saturated and the amount of residual austenite at the time of blanking increases, causing tempering during tempering, resulting in deterioration of impact toughness to be.

In the present invention, boron (boron, B) limits the content of boron to 0.0010-0.003% as an intergranular strengthening element for improvement of incombustibility and burden resistance. When the content is less than 0.0010%, the grain boundary strength enhancement effect due to grain boundary segregation due to segregation of boron atoms during heat treatment is insufficient and the graphitization promoting property is not sufficient in the treatment of graphite for improving cold formability, and 0.003% , The effect of saturation is saturated and the boron nitride is precipitated in the grain boundaries, resulting in deterioration of grain boundary strength.

The content of molybdenum (Mo) and tungsten (W) is limited to 0.01-0.5%. When the content is less than 0.01%, the effect of strengthening solubility in cementite and base metal in bainite is insufficient, and it is difficult to obtain the effect of improving the finish of stress. In order to finely distribute molybdenum precipitates during tempering, It is difficult. When the content exceeds 0.5%, the effect is saturated and the generation of low temperature structure (martensite + bainite) during production of wire rod is facilitated due to increase of the incombustibility and the heat treatment time during graphitization for improving cold formability This is because there is a drawback that it becomes longer.

The content of copper (Cu) is limited to 0.01-0.2%. If the content is less than 0.01%, the effect of improving the corrosion resistance is insufficient. If the content exceeds 0.2%, the improvement effect is saturated and the melting point of the grain segregation is lowered. And the 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 austenite crystal grain refinement effect is insufficient and the deposition distribution of titanium-based carbon and nitride effective for delayed fracture resistance is insufficient and it is difficult to expect the improvement effect. If the content exceeds 0.2% Is saturated and forms a coarse titanium nitride, which is detrimental to the fatigue characteristics.

The content of the cobalt is limited to 0.01-0.5%. If the content is less than 0.01%, the softening promoting effect is insufficient during spheroidization or graphitization heat treatment, which is the softening heat treatment for cold forging, and there is no effect on the diffusive hydrogen concentration in the grain boundary. If the content exceeds 0.5%, the effect is effective It is not preferable since the rate of softening is remarkably increased during the saturation and softening heat treatment, resulting in partial microstructure inhomogeneity in the heat treatment.

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

The reason for limiting the ferrite content on the complex structure to 5-25% in the present invention is as follows.

That is, when the ferrite structure fraction is less than 5%, the grain boundaries of the austenite grain boundaries for improving the delayed fracture resistance are attained to achieve discontinuity, and the amount of ferrite is too small to improve the effect. As the ferrite structure maintains continuity like the base metal structure, it is effective in reducing the yield strength but has a problem that the tensile strength is lowered. In the case of the anomaly treatment for obtaining the composite structure, the carbon concentration of austenite due to ferrite formation becomes too high, It is difficult to secure an effective bainite structure.

A more preferable ferrite structure fraction is in the range of 10-15%. That is, when the ferrite structure fraction in the final composite structure is 10-15%, it is possible to maximize the improvement effect of the delayed fracture resistance according to the resistance ratio (referred to as yield strength / tensile strength ratio), high strength, high elongation and homogeneous ferrite distribution It is because.

In the present invention, the reason for restricting the anomalous reverse heat treatment conditions to the temperature range of Ac3- (Ac3-Ac1) /1.3 to Ac3- (Ac3-Ac1) /5.5 is as follows in order to secure the ferrite structure fraction. Here, Ac3 represents the austenite transformation temperature at the time of heating, and Ac1 represents the transformation temperature to the anomaly (ferrite + austenite) upon heating. Ac3 and Ac1 transformation temperatures differ depending on the alloy component depending on the alloy component.

Ac3- (Ac3-Ac1) / 1.3 temperature is not preferable because it causes the ferrite generation amount to exceed 25% due to the abnormal heat treatment, If it exceeds 5.5, the amount of ferrite necessary for grain boundary discontinuity becomes less than 5%, and therefore, the effect is not expected to be expected.

(Ac3-Ac1) /1.7 to Ac3- (Ac3-Ac1) /2.5 in the range of more preferable range for the reverse heat treatment. This is a range considering the tensile strength tensile strength, discontinuity of old austenite grain boundary, discontinuity of grain boundary precipitates, bainite structure, elongation, decarburization control, and the like.

In the present invention, the isothermal heat treatment conditions for producing bainite structure after heating are limited to the range of martensitic transformation temperature (Ms) and normal Ms + (150 ± 30 ° C).

That is, at a temperature below Ms + 120 ° C, the thickness of the cementite is thinner in the bainite structure, and the ferrite distribution layer in the base material is very finely distributed together with the cementite so that the cementite and the ferrite start to deform at the time of tensile strain, If the temperature is higher than Ms + 180 ° C, the strength of the tensile strength can not be increased due to pro-eutectoid ferrite transformation and pearlite transformation, and the yield strength and impact toughness are rapidly lowered.

The more preferable bainite transformation temperature range having a high strength and a low resistance is Ms + (150 ± 10 ° C). This is because the yield ratio, the vibration resistance, the bolt fastening strength, the corrosion resistance, the stress relaxation at the bolt fastening, the impact toughness, the elongation, The range considered.

Hereinafter, the present invention will be described more specifically by way of examples.

Example

The inventive materials (1-8) satisfied the composition range of the present invention, and the comparative materials (1-4) were outside the scope of the present invention. The prepared steels were cast with 50 kg of ingot and subjected to homogenization heat treatment at 1250 ° C. for 48 hours to be hot rolled to a thickness of 13 mm. At this time, the finishing temperature was 950 ° C or higher, followed by hot rolling and air cooling, and the rolling ratio was 80% or more.

C Si Mn Cr V Ni Mo Ti W B P S N ₂ Inventory 1 0.45 3.03 0.29 0.58 0.05 - - - - - 0.005 0.004 0.008 Inventory 2 0.40 3.42 0.31 0.79 0.2 - - 0.01 - 0.0013 0.006 0.005 0.014 Inventory 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 Article 5 0.44 3.96 0.23 0.27 0.06 - - - 0.2 0.0015 0.008 0.008 0.008 Inventions 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 8 0.44 3.1 0.34 0.55 0.07 - - - - - 0.007 0.006 0.008 Comparison 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 Comparison 4 0.20 0.20 0.80 0.72 - - 0.04 - - 0.0015 0.009 0.004 0.005

Test specimens for evaluating mechanical properties (tensile and impact properties) and delayed fracture resistance were taken from the hot rolled materials in the rolling direction of the rolled material.

At this time, the heat treatment conditions were the heating and isothermal heat treatment conditions shown in Tables 2 and 3 below. In addition, Ms transformation temperatures, Ac1 and Ac3 were measured using a dilatometry, and the results are shown in Tables 2 and 3 below.

Used steel grade Heating temperature (占 폚) Ac3 - [(Ac3-Ac1) / X] Heating temperature (占 폚) Heating time (min) Isothermal heating (° C) (Ms + X) Tempering temperature (° C) · Isothermal holding time (min) · Tempering time (min) Ferrite phase fraction (%) Transformation temperature Ac3 Ac1 Ms Honorable 7 Inventory 2 X = 2 30 X = 140 40 17 955 833 290 Honors 8 Inventory 3 X = 2 70 X = 140 40 10 883 803 229 Proposition 9 Invention 4 X = 2 80 X = 140 40 8 880 782 260 Inventory 10 Invention Article 5 X = 2 30 X = 140 40 20 961 842 288 Exhibit 11 Inventions 6 X = 2 40 X = 140 40 12 899 817 250 Inventory 12 Invention 7 X = 2 120 X = 140 40 8 857 775 208 Comparative Example 9 Comparison 1 900 30 Tempering temperature = 450 60 0 - - - Comparative Example 10 Comparative material 2 900 30 Tempering temperature = 450 60 0 Comparative Example 11 Comparative material 3 950 30 Tempering temperature = 450 60 0 Comparative Example 12 Comparison 4 900 30 Tempering temperature = 450 60 0

Inventive example 1-6 in the above Table 2 shows that the temperature of 830-890 deg. C in the range of 815 deg. C to 915 deg. C, which is the abnormal reverse heating temperature range of the same alloy component (invention material 1) The isothermal heat treatment temperature range for the bainite transformation is set to be Ms + (150 ~), which is prepared so that the ferrite phase fraction ranges from 5 to 25% in the range of Ac3 - [(Ac3-Ac1) / (1.3-5.5) 30 占 폚) at a cooling rate of 70 占 폚 / sec or more and then heat-treated for 40 minutes. Here, Ms is the martensitic transformation start temperature.

In Comparative Example 1-4, the ferrite phase fraction was prepared in the same manner as in the case of Inventive Material 8, with the abnormally reversed heating temperature being Ac3 - [(Ac3-Ac1) / (1.3-5.5) And was subjected to heat treatment for 40 minutes under isothermal heating temperature Ms + 180 ° C and Ms + 120 ° C for 40 minutes.

On the other hand, in Comparative Example 5-8, a conventional tempered martensite structure having a ferrite phase fraction of 0% in the same alloy component (Inventive Material 8) was prepared.

In Inventive Examples 7-12 in Table 3 above, each of the ferrite phase fractions was heated at a temperature of Ac3- [(Ac3-Ac1) / 2)], which is an abnormal intermediate temperature range of each alloy component, -25% and quenched at a cooling rate of 70 C / sec or more to Ms + 170 for bainite transformation, maintaining the isothermal holding time for 40 minutes and then cooling with oil. On the other hand, Comparative Example 9-12 was heated in the range of 800-950C in the austenite single phase region using the comparative member 1-4, tempered at 450C.

In order to evaluate the tensile and impact properties of the prepared materials, the tensile test specimens were subjected to the KS standard (KS B 0801) No. 4 test specimen and the tensile test was carried out at a crosshead speed of 5 mm / min . Impact test specimens were prepared in accordance with KS standard (KS B 0809) No. 3 test piece, in which the notch direction was machined in the side direction (LT direction) in the rolling direction. The microstructure fraction was examined using a point counting method, which is a general optical microscopic measurement method, wherein the surface to be examined was 1000 mm ² .

Tensile properties and impact toughness of the prepared test pieces according to the inventive and comparative examples were measured and the results are shown in Tables 4 and 5 below.

On the other hand, microstructural differences between the inventive example 2 and the comparative example are shown in Fig. 1, whereas the inventive example 2 is a bainite structure (black area is a ferrite structure and gray area is a bainite structure) It was a typical tempered martensite structure.

Tensile strength (kg / mm ² ) Yield strength (kg / mm ² ) Elongation (%) Section reduction rate (%) Impact Toughness (J / cm ² ) Yield ratio Inventory 1 140 85 24 47 47 0.61 Inventory 2 145 72 22 40 40 0.50 Inventory 3 147 67 18 42 30 0.46 Honorable 4 150 50 15 40 20 0.33 Inventory 5 142 69 23 40 42 0.49 Inventory 6 125 45 14 40 35 0.37 Comparative Example 1 144 105 33 50 60 0.73 Comparative Example 2 155 35 5 20 8 0.23 Comparative Example 3 150 35 6 22 10 0.23 Comparative Example 4 136 100 15 60 140 0.74 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 (%) Section reduction rate (%) Impact Toughness (J / cm ² ) Yield ratio Honorable 7 154 90 22 45 42 0.58 Honors 8 168 99 19 55 41 0.59 Proposition 9 160 93 25 43 38 0.58 Inventory 10 158 95 19 45 34 0.60 Exhibit 11 161 96 22 50 40 0.60 Inventory 12 169 100 20 44 30 0.59 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 Tables 4 and 5, the yield ratios of the inventive examples are in the range of 0.33 to 0.61, and the tensile strength is in the range of 125 to 165 kg / mm 2. In Comparative Examples, the yield ratio is 0.70 to 0.94 and the tensile strength is 110 to 225 kg / Mm < 2 >, or in Comparative Examples 2 and 3, the yield ratio is as low as 0.23. However, considering the poor elongation and impact toughness, it can not be considered effective yield reduction.

As described above, the present invention exhibits excellent mechanical properties and a low yield ratio, so that the present invention using ferrite and bainite composite textures can remarkably improve the yield ratio while having the same strength as that of the comparative example Able to know. In order to exhibit the effect of the present invention, the bainite transformation temperature is most preferably in the range of Ms + (150 ± 10 ° C).

As shown in FIG. 1, the effect of the present invention is that ferrite in the base material has a continuous distribution shape in the ferrite + bainite complex structure, so that the ferrite, which is a soft structure, is preferentially deformed during tensile deformation, / Mm < 2 >, it is possible to secure a low resistance ratio showing a low yield strength.

As described above, the present invention proposes a bainite steel alloy component and a heat treatment condition capable of enhancing the tensile strength while having a strength characteristic of low resistance, so that the strength of the bolt can be secured at the same time while attaining the resistance reduction of the bolt It is possible to provide a ferrite + bainite composite structure steel for a high strength resisting bolt.

Claims (5)

A method for manufacturing a bolt steel, Wherein the composition contains 0.4-0.60% of carbon, 2.0-4.0% of silicon, 0.2-0.8% of manganese, 0.25-0.8% of chromium, 0.01% or less of phosphorus, 0.01% or less of sulfur, 0.005-0.01% , And a mixture of 0.05-0.2% of vanadium, 0.05-0.2% of niobium, 0.3-2.0% of nickel, 0.001-0.003% of boron, 0.01-0.5% of molybdenum, 0.01-0.2% of titanium, 0.01-0.5% of tungsten, (Ac3-Ac1) /1.3 selectively containing one or more elements selected from the group consisting of Fe3O3, Fe3O3, Fe3O3, Fe3O3, Fe3O3, (150 ° C. ± 30 ° C.) at a cooling rate of 70 ° C./sec or more after the ferrite phase fraction is 5-25% by heating for 20 minutes or more within the range of , Followed by isothermal heat treatment for 20 minutes or more, followed by oil cooling or air cooling. The composite structure steel for high strength bolts The method according to claim 1, Wherein the silicon is contained in a range of 2.8-3.3%. The method of manufacturing a composite structure steel for a high strength bolt The method according to claim 1, Wherein the heat treatment temperature is a heat treatment in a temperature range of Ac3- (Ac3-Ac1) /1.7 to Ac3- (Ac3-Ac1) /2.5. The method according to claim 1, Wherein the ferrite phase fraction is limited to a range of 10-15%. The method according to claim 1, Wherein the isothermal heating temperature is limited to a range of Ms + (150 占 10 占 폚).