KR20000042531A - Method for producing bolt having high toughness - Google Patents

Abstract

PURPOSE: A method is provided to produce a bolt improving high toughness and delayed fracture resistance as well as having high strength with a low surface decarbonization layer. CONSTITUTION: A steel billet is composed of 0.40 to 0.60wt% of carbon, 2.0 to 4.0wt% of silicon, 0.2 to 0.8wt% of manganese, 0.2 to 0.8wt% of chrome, less than 0.01wt% of phosphorus, less than 0.01wt% of sulfur, 0.005 to 0.03wt% of nitrogen, less than 0.005wt% of oxygen and the remain of iron and incidental impurities. Herein, the steel billet contains one or two selected among a group consisting of 0.05 to 0.2wt% of vanadium, 0.05 to 0.2wt% of niobium, 0.3 to 2.0wt% of nickel, 0.001 to 0.003wt% of boron, 0.01 to 0.5wt% of molybdenum, 0.01 to 0.2wt% of titanium, 0.01 to 0.5wt% of tungsten, 0.01 to 0.2wt% of copper and 0.01 to 0.5wt% of cobalt. The steel billet is heated and rolled to be a regular shaped bolt. The bolt is also heated more than for 20minutes and suddenly cooled at 70°C/sec of speed. The billet is isothermally heated more than for 20minutes and cooled by fluid or air to be a bolt having high toughness.

Description

Manufacturing method of high toughness bolt

The present invention relates to a method of manufacturing bolts, and more particularly, to a method of manufacturing bolts excellent in toughness by having a small surface decarburization layer and having a bainite structure.

High strength bolts are required for fastening members for efficient construction of steel structures, weight reduction, multifunction, and high performance of automobile parts. However, since the high strength of the bolt causes the deterioration of the delayed fracture resistance and the deterioration of the toughness, it is currently impossible to use the tensile strength of 130kg / mm 2 or more, which limits its use and range.

Most existing bolts are quasi single phase structure of tempered martensite. Carbide-based precipitates are distributed at grain boundaries, and the base metal has precipitates in ras martensite. However, this precipitate acts as a trapped site of hydrogen (invading from the outside) that inhibits the high-strength of the bolt, deteriorating grain boundary strength and lowering the delay fracture resistance.

Therefore, it is desirable to achieve high toughness while suppressing the distribution of grain boundary precipitates to the maximum and to achieve high toughness by improving crack propagation resistance or fracture resistance. The followings are expected benefits when developing bolted steel with high delay fracture resistance and high strength.

In the aspect of steel structure, bolt fastening does not require more skilled technique than welding joint, and considering the advantages of replacing weak welds, firstly, it is possible to increase the stability of steel structure by strengthening the fastening force during bolt fastening, and secondly, to reduce the number of bolt fastening. It is possible to reduce the amount of steel used. In addition, the third aspect of the vehicle parts, contributes to the light weight of the component, and fourth, there is an advantage that can be diversified design and compact (compact) of the vehicle assembly apparatus according to the weight of the component.

Conventional techniques for improving delayed fracture resistance include: 1) suppressing corrosion of steel, 2) minimizing hydrogen infiltration, 3) suppressing the accumulation of diffusible hydrogen contributing to delayed fracture, and 4) limiting diffusion hydrogen concentration. The use of large steels, 5) minimizing tensile stress, and 6) mitigating stress concentration. As a means to achieve this, it is a situation to mainly use a method to pursue high alloying or to give a surface coating or plating to prevent external hydrogen intrusion. As can be seen through this, there is currently no way to fundamentally solve the problem by improving the temper martensite organization, which is a direct cause of the high strength.

The present invention is to provide a method of manufacturing a bolt that can be high strength and can prevent the degradation of the toughness and delayed fracture resistance accordingly, an object thereof.

The bolt manufacturing method of the present invention for achieving the above object, in weight%, carbon: 0.40-0.60%, silicon: 2.0-4.0%, manganese: 0.2-0.8%, chromium: 0.2-0.8%, phosphorus: 0.01% or less , Sulfur: 0.01% or less, nitrogen: 0.005-0.03%, oxygen: 0.005% or less, including vanadium: 0.05-0.2%, niobium: 0.05-0.2%, nickel: 0.3-2.0%, boron: 0.001- At least one selected from the group consisting of 0.003%, molybdenum: 0.01-0.5%, titanium: 0.01-0.2%, tungsten: 0.01-0.5%, copper: 0.01-0.2%, and cobalt: 0.01-0.5% The steel billet, which is optionally contained and composed of the remaining Fe and other unavoidable impurities, is heated to Ac ₁ at a heating rate of 15 ± 5 ° C./min, and an abnormal range of Ac ₁ to Ac ₃ is heated to 6 ± 3 ° C./min. After heating at a rate of up to 1050 ± 50 ℃ at a heating rate of 10 ± 5 ℃ / min, maintained for 30-60 minutes and then rolled the wire rod, the wire is machined into a bolt of a certain shape, and then the processed bolt is Ac ₃ transformation point this Heating at a temperature of at least 20 minutes, quenching the martensite transformation start temperature (Ms) at a cooling rate of at least 70 ° C / sec to a temperature of + 30 ° C to Ms + 90 ° C and isothermally heating at this temperature for at least 20 minutes, followed by oil cooling or It is comprised including air cooling.

Hereinafter, the present invention will be described in detail.

The present inventors have reviewed the entire process of bolt manufacturing to manufacture bolts capable of high strength and prevent the degradation of the toughness and delayed fracture resistance, and as a result, the manufacturing process of the wire rod and the heat treatment of the processed bolts in relation to the steel component system Experiments confirmed that the solution can be solved by comprehensively controlling the conditions, and the present invention has been completed.

That is, the present invention suppresses surface decarburization as much as possible in the wire rod manufacturing process, which is a bolt material, and heat-processes the processed bolt to bainite, thereby inducing hydrogen in the presence of grain boundary carbides due to bainite properties. In addition to improving the problem of intergranular embrittlement according to the present invention, there is a feature to improve the delayed fracture resistance and toughness due to the reduction of the surface decarburization layer. The present invention is achieved by the organic combination of the steel component and the manufacturing process, which will be described in detail below.

[Steel Ingredients]

The carbon content of 0.40-0.60% is difficult to obtain sufficient tensile strength and yield strength as a high strength bolt after heat treatment for bainite production at 0.40% or less, and toughness after heat treatment at 0.60% or more. Hardening and hardening of cracks due to plate martensite formation during hardening. Permanent deformation, fatigue characteristics, carbonitride distribution, bainite structure, bainite transformation during decarburization and bolt fastening This affects the time required.

Limiting the content of silicon (Si) to 2.0-4.0% is less than 2.0%, it is difficult to secure the strength due to the insufficient solid-solution strengthening effect of ferrite in bainite structure, and also delayed fracture resistance, surface corrosion characteristics, impact toughness , The structure of bainite structure, permanent deformation during bolting, etc. Also, due to difficult distribution of surface ferrite decarburized layer in wire heating furnace for wire decarburization control, decarburization is intensified and hardenability during wire cooling increases This is because it is difficult to control the surface scale characteristics of the furnace. Above 4.0%, the above-mentioned effects are not preferable because they saturate and affect hardenability, bainite structure, impact toughness, fatigue characteristics, and the like, and silicon during the manufacture of parts or billets for wire rod manufacturing This is because the quality of the final product is degraded due to the inhomogeneity of the microstructure due to segregation, 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 silicone component range in the present invention is 2.8-3.3%. These include isothermal heat treatment time and residual austenite fraction to produce bainite structure, high strength and high toughness of bainite, delayed fracture resistance (diffuse hydrogen content, control of precipitation of critical precipitates), surface decarburization, stress relaxation after bolting (stress relaxation) or permanent strain resistance, dynamic and static fatigue characteristics can be considered very effectively.

The reason why the content of manganese (Mn) is 0.2-0.8% is an element that forms a solid solution to form a solid solution to strengthen the solid solution, and is very useful for high-strength bolt characteristics. Tissue heterogeneity caused by simple stones has a more detrimental effect on bolt properties. Macro sedimentation and micro segregation tend to occur depending on the segregation mechanism during steel coagulation. Manganese segregation promotes segregation due to its relatively low diffusion coefficient compared to other elements, and the improvement of hardenability is caused by core martensite. ) Is the main reason for generating When manganese is added below 0.2%, segregation zone is hardly formed by manganese segregation, but stress relaxation improvement effect by solid solution strengthening is hardly 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 more than 0.8%, local anisotropy is increased due to manganese segregation and formation of segregation zones. That is, the bolt properties are degraded due to the heterogeneity of the tissue. 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 reason that the content of chromium (Cr) is 0.20-0.8% is that below 0.25%, it is difficult to form the surface ferrite layer for surface decarburization control during heat treatment of high silicon-added steel, so there is almost no decarburization inhibitory effect and expects improvement of quenchability. Because it is difficult. Above 0.8%, it is not preferable because the transformation time of bainite is longer during isothermal heat treatment, and it is difficult to generate a proper surface ferrite layer during charging by wire heating to control wire decarburization layer, which affects homogeneous decarburization control. .

If the content of phosphorus (P) and sulfur (S) is 0.01% or less, the upper limit is limited to 0.01% because phosphorus segregates at grain boundaries and degrades toughness. It is preferable to limit the upper limit to 0.01% because it lowers and forms an emulsion, which adversely affects the delayed fracture resistance and the stress relaxation characteristics.

The content of nitrogen (N) is 0.005-0.01% because it is difficult to form vanadium and niobium-based nitrides that act as trap sites for non-diffusive hydrogen below 0.005%, and the effect is saturated at 0.03% or more. to be.

The content of oxygen (O) is 0.005% because coarse oxide-based nonmetallic inclusions are easily formed at 0.005% or more, thereby reducing fatigue life.

Vanadium (V) or niobium (Nb) is an element that improves delayed fracture resistance and stress relaxation. The content of 0.05-0.2% is less than 0.05%, which leads to less diffusion of vanadium or niobium-based precipitates in the base material. It is difficult to expect the effect of improving the delayed fracture resistance due to insufficient role as a hydrogen trap site, and it is difficult to expect the precipitation strengthening effect, and the improvement effect on the stress relaxation resistance is not sufficient, and it is expected to refine the austenite grain. Difficulty affects the bainite organization. Above 0.2%, the improvement effect on delayed fracture resistance and stress relaxation resistance due to precipitates is saturated, and coarse alloy carbides, which are not dissolved in the base metal, increase in the austenite heat treatment to act like nonmetallic inclusions. This is because it causes a decrease.

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. Its content of 0.3-2.0% is less than 0.3%, so it is difficult to expect improvement effect of delayed fracture resistance due to incomplete formation of surface thickening layer, 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 effect of cold forming during cold bolt processing, and the effect is saturated at 2.0% or more, and it causes temper brittleness during tempering due to the increase of the amount of retained austenite when quenched, resulting in a decrease in impact toughness. .

Boron (boron, B) is a grain boundary strengthening element for improving the hardenability and delayed fracture resistance in the present invention, the content of boron is 0.001-0.003%, below 0.0010%, the grain boundary strengthening according to the grain boundary segregation of boron atoms during heat treatment. This is because the effect of improving grain boundary strength is inadequate, and the graphitization promoting effect is insufficient when the graphitization treatment is performed to improve cold formability. At 0.003% or more, the effect is saturated. This is because precipitation of causes a decrease in grain boundary strength.

Molybdenum (Mo) and tungsten (W) content in the range of 0.01 to 0.5% is less than 0.01%. When tempered (whenever), cementite inhibits the growth of cementite when it grows in transition from grain carbide, This is because it is difficult to obtain an improvement effect, and it is difficult to secure a stable structure at high temperature by finely distributing molybdenum precipitates during tempering. At 0.5% or more, the effect is saturated, and the increase in hardenability makes it easy to form low-temperature structure (martensite + bainite) during wire rod manufacturing, and the heat treatment time is increased during the graphitization treatment to improve the cold forming property. Because.

In the case of the copper (Cu) content of 0.01-0.2%, the improvement effect on the corrosion resistance is insufficient at 0.01% or less, and the improvement effect is saturated at the 0.2% or more, and the melting point is lowered at the grain boundary segregation. This is because the surface flaw is more likely to occur in grain boundary embrittlement when charging the furnace for wire rolling, and the impact toughness in the final product is lowered.

Titanium content of 0.01-0.2% is less than 0.01% because the effect of miniaturization of austenite crystal grains is insufficient, and the precipitation distribution of titanium-based carbon and nitride, which is effective for delayed fracture resistance, is insufficient. This is because at 0.2% or more, the effect is saturated and coarse titanium nitride is formed, which is detrimental to the fatigue properties.

If the cobalt content is less than 0.01%, the softening effect is not sufficient in spheroidization or graphitization heat treatment, which is a material soft nitriding heat treatment for bolt processing, and there is no effect on the grain boundary diffusive hydrogen concentration. The effect is saturated and the soft nitriding rate during softening heat treatment may increase dramatically, resulting in partial microstructure heterogeneity during heat treatment, which is undesirable.

[Manufacture process]

The steel billet formed as described above is (1) hot wire rolled to make a wire, then (2) the wire is processed into a bolt of a certain shape and the bolt is manufactured by a series of processes of heat-treating the bolt. In the present invention, the thickness of the surface decarburization layer in the hot wire rolling process during such a process is reduced, and the final structure of the bolt processed with this material is controlled by bainite, thereby improving toughness and critical delay fracture resistance.

(1) wire manufacturing process

First, the steel billet is heated at a heating rate of 15 ± 5 ° C./min to Ac ₁ (ideal transformation transformation start temperature). When the heating rate up to Ac ₁ is less than 10 ° C / min, the decarburization reaction proceeds heterogeneously due to the increase in the amount of oxidation and the distribution of the heterogeneous oxide layer, thereby making it difficult to obtain the effects of the present invention. If / min is exceeded, it is not preferable because the temperature deviation inside and outside the billet is deepened and billet bending occurs.

Then, heating is performed at a heating rate of 6 ± 3 ° C./min from the ideal range Ac ₁ to Ac ₃ . If the temperature increase rate is less than 3 ℃ / min when passing the above ideal temperature range, the thickness of the surface ferrite layer is increased more than necessary, rather it is difficult to obtain the decarburization improvement effect, and also has a disadvantage of long reheat time (charge time). . If the temperature increase rate exceeds 9 ℃ / min, it is difficult to form the appropriate surface ferrite layer required for suppressing the decarburization reaction.

Then, the wire is heated to 1050 ± 50 ℃ at a heating rate of 10 ± 5 ℃ / min and maintained for 50-70 minutes before rolling the wire. When the temperature rise rate is less than 5 ° C / min from Ac ₃ temperature, which is the abnormal temperature end temperature, to the holding temperature of 1050 ± 50 ° C, the shelf time increases. When the temperature exceeds 15 ° C / min, the billet is increased due to the deep temperature deviation inside and outside the billet. Warpage is likely to occur. In addition, if the holding temperature is less than 1000 ℃, there is a problem in controlling the proper thickness of the billet surface ferrite layer for decarburization control, it is not easy to re-use the coarse precipitated vanadium-based or niobium-based precipitates during the manufacture of billet, increase the hot deformation resistance When rolling, the workability is poor due to overload. On the other hand, when it exceeds 1100 ° C, the decarburization control ferrite layer cannot be deposited on the surface, which is not preferable. That is, it is possible to have a surface ferrite layer with a very low carbon solubility. However, if the heating temperature exceeds 1100 ° C, the surface ferrite layer is rapidly decarburized because the surface ferrite layer transforms into austenite, thereby increasing the surface decarburization. there is a problem. In addition, if the heating and holding time is less than 30 minutes, it is difficult to secure a uniform temperature distribution inside the billet for the wire rod rolling, but if it exceeds 60 minutes, the oxidation amount is rapidly increased to provide a suitable ferrite layer for controlling the billet surface decarburization. Difficult to secure

When the heating rate of the steel billet is changed as described above, the ferrite surface decarburization layer having a thickness of about 0.02-0.05 mm may be formed on the surface of the wire rod. If the thickness of the ferrite surface decarburization layer is too thin, the effect of improving delayed fracture resistance after bolt manufacturing is insufficient. If the thickness of the ferrite surface decarburization layer is too thick, it is difficult to secure a uniform ferrite layer and both tensile strength and yield strength are reduced.

(2) Bolt processing and bolt heat treatment

The wire rod thus rolled is processed into a bolt having a predetermined shape, and then the processed bolt is heat-treated so that the final structure is bainite.

In the heat treatment of the present invention, first, the processed bolt is cracked by heating the bolt manufactured as described above to the temperature above the Ac ₃ transformation point, which is an austenite single phase and sufficient aberration in the abnormal region where the ferrite and austenite phase coexist below the Ac ₃ transformation point. This is because the austenitization may not be performed, which may later lead to a heterogeneity of bainite fabrication. This reheating temperature is preferably carried out at as low as 1050 ° C. The reason for this is that surface decarburization and coarsening of austenite grains occur when heating the material, so that the bainite structure composed of quality characteristics and coarse cementite in the final product is formed, so that the impact toughness, tensile and yield strength, stress relaxation, fatigue This is because it is harmful to characteristics and the like. It is maintained at the above temperature for 20 minutes or more because it is difficult to secure a complete bainite structure because sufficient austenitization is not achieved when the heating time is insufficient.

After heating, it is cooled to a temperature (isotherm heat treatment temperature: Ms + 30 ° C to Ms + 90 ° C) immediately above the martensite transformation temperature (Ms) at a cooling rate of 70 ° C / sec or more, and isothermally heat treated at this temperature range. . If the cooling rate at this time is 70 ℃ / sec or less, abnormal tissue may occur, it is difficult.

The isothermal heat treatment temperature is preferably Ms + 30 ° C. to Ms + 90 ° C., which is not preferable because the bainite transformation time is longer at Ms + 30 ° C. and martensite is more likely to occur when temperature deviations occur. , Elongation and impact toughness are reduced, and at Ms + 90 ℃ and above, there is a problem in securing proper yield strength due to the drastic reduction of yield ratio (yield strength / tensile strength ratio), resulting in poor stress relaxation during bolting. This is because the impact toughness is deteriorated due to the decrease in impact toughness, and also affects the critical delay fracture strength and the fatigue characteristics.

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

EXAMPLE

50kg billet having the composition shown in Table 1 below was heated under the conditions of Table 2 and rolled wire to prepare a wire of 13mm diameter. The depth of the surface decarburized layer was measured according to the standard of KS D 0216 for this wire rod, and the results are shown in Table 2. At this time, the measurement position was measured at the position where the wire rod section was divided into 8 and the measured value was based on the average value.

C Si Mn Cr V Nb Mo Ti W B P S N ₂ Inventive Steel 1 0.45 3.03 0.29 0.58 0.54 - - - - - 0.005 0.004 0.008 Inventive Steel 2 0.40 3.42 0.31 0.79 0.2 - - 0.01 - 0.0013 0.006 0.005 0.014 Invention Steel 3 0.60 2.99 0.32 0.33 0.05 0.54 - - 0.02 - 0.007 0.009 0.007 Inventive Steel 4 0.45 2.0 0.77 0.51 0.11 - 0.2 0.03 - - 0.006 0.008 0.009 Inventive Steel 5 0.44 3.96 0.23 0.27 0.06 - - - 0.2 0.0015 0.008 0.008 0.008 Inventive Steel 6 0.53 3.01 0.35 0.55 - - 0.05 0.05 0.07 0.0010 0.004 0.009 0.004 Inventive Steel 7 0.58 2.56 0.60 0.29 - 1.10 0.13 0.10 - - 0.005 0.006 0.005

Wire number Wire heating furnace heating pattern Wire Rod Characteristics (mm) Steel grade Heating rate up to Ac ₁ (℃ / min) Ideal temperature rise rate (℃ / min) Temperature rise rate after Ac ₃ (℃ / min) Heating temperature (℃) Holding time (min) Total decarburization (mm) Ferrite Layer Thickness (mm) One 15 4 11 1050 30 0.04 0.03 Inventive Steel 1 2 15 5 13 1100 60 0.04 0.03 Inventive Steel 2 3 15 6 12 1000 30 0.05 0.04 Invention Steel 3 4 16 7 12 1050 60 0.06 0.05 Inventive Steel 4 5 17 8 12 1050 50 0.05 0.04 Inventive Steel 5 6 15 10 11 1050 50 0.06 0.04 Inventive Steel 6 7 15 10 13 1100 60 0.07 0.05 Inventive Steel 7 a 16 12 12 1050 50 0.10 0.08 Inventive Steel 1 b 16 15 12 1050 40 0.12 0.10 Inventive Steel 2 c 16 15 12 1050 60 0.13 0.11 Invention Steel 3 d 15 17 11 1100 30 0.17 0.14 Inventive Steel 4 e 15 17 11 1050 50 0.18 0.15 Inventive Steel 5 f 15 19 11 1100 30 0.19 0.15 Inventive Steel 6 g 15 19 11 1050 50 0.20 0.16 Inventive Steel 7

As can be seen from Table 2, the wire rod (1-7) prepared the conditions of the present invention shows a surface decarburization depth range of 0.04-0.06mm, while the wire rod (ag) outside the conditions of the present invention is 0.10 The wire decarburization was shown in the range of -0.20mm. That is, in the present invention, as the ferrite layer is properly formed and the wire depth of which the total depth of the decarburized layer is significantly reduced is used, as shown in the following results, it is very advantageous for characteristics such as delayed fracture of the bolt.

As described above, the wire decarburization layer having different surface decarburization layers (1-7, a-g) was processed into bolts, and the bolts were then subjected to heat treatment under the conditions shown in Table 3 below.

Target Wire Rod Heating condition Isothermal Heat Treatment Condition Steel grade (Ms transformation temperature) Temperature (℃) Time (min) Temperature (℃) Holding time (min) 1, a 950 30 Ms + 60 30 Inventive Steel 1 (290 ℃) 2, b 1000 40 Ms + 60 40 Inventive Steel 2 (307 ℃) 3, c 1000 40 Ms + 60 40 Invention Steel 3 (247 ℃) 4, d 1030 40 Ms + 60 40 Inventive Steel 4 (278 ℃) 5, e 1030 40 Ms + 60 40 Inventive Steel 5 (304 ℃) 6, f 1030 40 Ms + 60 40 Inventive Steel 6 (268 ℃) 7, g 1050 40 Ms + 60 40 Inventive Steel 7 (223 ℃)

Tensile, impact, and delayed fracture characteristics of the materials prepared as described above were evaluated as follows, and the results are shown in Tables 4 and 5.

(1) Tensile strength was measured at 5 mm / min cross head speed using KS standard (KS B0801) No. 4 test piece.

(2) Impact toughness was prepared in accordance with the KS standard (KS B 0809) No. 3 impact test, the notch direction was machined from the side of the rolling direction (L-T direction).

(3) For the evaluation of the delayed fracture resistance, the constant load method which is generally used was applied. This evaluation method is a general method for evaluating delayed fracture resistance in terms of additional stress or the time required to break under a specific stress. In the delayed fracture test, the test stress was determined based on the notched tensile strength.

At this time, the delayed fracture tester used a constant loading type delayed fracture testing machine. The delayed fracture test specimen was manufactured with a specimen diameter of 5 mm, a notch diameter of 4 mm, and a notch root radius of 0.1 mm. The test piece atmosphere solution was performed at room temperature (25 ± 5 ° C.) of pH 2 ± 0.5 with a whirlpool buffer solution (HCl + CH ₃ COONa).

(4) The critical delay fracture strength means the tensile strength at which the time from failure to failure at the same stress ratio (load stress / notch tensile strength ratio, 0.5) is not broken up to 150 hours or more, and the notch strength is a tensile test of the notched test piece. The maximum load ÷ notched area was obtained. The number of specimens for setting the critical delay fracture strength was based on the case where more than 13 undecided specimens were used.

division Tensile Strength (Kg / ㎡) Yield strength (kg / ㎡) Elongation (%) Cross section reduction rate (%) Impact Toughness (J / ㎠) Critical Delay Break Strength (kg / ㎡) Critical Delay Failure Stress Ratio Target Wire Rod Invention One 140 105 15 63 152 150 0.65 One 2 162 127 15 62 148 150 0.60 2 3 175 136 16 63 155 150 0.60 3 4 160 128 15 61 148 150 0.60 4 5 160 133 15 65 154 150 0.60 5 6 164 135 15 62 155 150 0.60 6 7 173 139 12 62 130 150 0.55 7

division Tensile Strength (Kg / ㎡) Yield strength (kg / ㎡) Elongation (%) Cross section reduction rate (%) Impact Toughness (J / ㎠) Critical Delay Break Strength (kg / ㎡) Critical Delay Failure Stress Ratio Target Wire Rod Comparative material One 119 89 13 65 156 145 0.5 a 2 137 107 15 63 150 145 0.5 b 3 148 115 15 65 155 145 0.5 c 4 136 108 14 62 149 145 0.5 d 5 136 113 13 66 150 145 0.5 e 6 139 114 13 63 152 145 0.5 f 7 147 118 12 60 130 145 0.5 g

As shown in Table 4 and Table 5, the impact toughness of the present invention exhibits a 130-155 J / cm 2 range and the critical delay fracture strength is 150 kg / mm 2, while the comparative materials have a threshold toughness of 130-150 J / cm 2 and are critical. The present invention using the bainite structure at a delayed breakdown strength of 145Kg / mm 2 can be seen that the high toughness and the critical delayed breakdown strength are remarkably improved compared to the comparative materials.

As described above, the present invention proposes an alloy composition system, wire rod manufacturing conditions, and heat treatment conditions of bainite steel in order to improve the excellent impact toughness and the resistance to delayed fracture resistance, thereby achieving high strength and high toughness of the bolt while simultaneously achieving excellent delayed fracture resistance. As it can be secured, it is possible to provide bainite steel for high strength bolts.

Claims (2)

By weight, carbon: 0.40-0.60%, silicon: 2.0-4.0%, manganese: 0.2-0.8%, chromium: 0.2-0.8%, phosphorus: 0.01% or less, sulfur: 0.01% or less, nitrogen: 0.005-0.03% , Oxygen: 0.005% or less, 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 at least one or two or more selected from the group consisting of the remaining Fe and other unavoidable impurities Heating the steel billet to Ac ₁ at a heating rate of 15 ± 5 ℃ / min, and heating the ideal range Ac ₁ to Ac ₃ at a heating rate of 6 ± 3 ℃ / min and then heating speed to 1050 ± 50 ℃. After heating to ± 5 ℃ / min, hold for 30-60 minutes and roll the wire, process the wire into a bolt of certain shape, and then heat the processed bolt for more than 20 minutes at the temperature above Ac ₃ transformation point and over 70 ℃ / sec. Cold Speed and a rapid cooling to the martensitic transformation starting temperature (Ms) + 30 ℃ ~Ms + 90 ℃ temperature and comprises to isothermal heating, and then, yunaeng or air-cooling over 20 minutes at this temperature process for producing a waterborne bolt 2. The method of claim 1, wherein the bolt has a surface decarburization layer of 0.02-0.05 mm and an interior of bainite structure.