KR20000033851A - Composite composition steel for high strength and high percentage of bolt having good delayed fracture resistance and preparation method thereof - Google Patents

Abstract

PURPOSE: A composite composition steel for bolt and a preparation method thereof are provided which provide the bolt having high strength, high percentage of elongation and good delayed fracture resistance. CONSTITUTION: The composite composition steel for bolt comprises: (i) 0.4-0.60 wt% of carbon, 2.0-4.0 wt% of silicone, 0.2-0.8 wt% of manganese; 0.25-0.8 wt% of chromium, less than 0.01 wt% of sulfur, less than 0.01 wt% of phosphorous, 0.005-0.01 wt% of nitrogen and less than 0.005 wt% of oxygen; (ii) at least one selected from 0.05-0.2 wt% of vanadium, 0.05-0.2 wt% of niobium, 0.3-2.0 wt% of nickel, 0.001-0.003 wt% of boron, 0.01-0.5 wt% of molybdenum, 0.01-0.2 wt% of titanium, 0.01-0.5 wt% of tungsten, 0.01-0.2 wt% of copper and 0.01-0.5 wt% of cobalt; and (iii) the remained of Fe and inevitable impurities, wherein the fine composition has the composite composition of ferrite and bainite and the phase proportion of the ferrite is 5-25%. The method comprises steps of: (i) controlling the phase proportion of the ferrite at 5-25% and the phase proportion of the austenite at 75-95% by heating the composite composition steel within the range of from Ac3-(Ac3-Ac1)/1.3 to Ac3-(Ac3-Ac1)/5.5 for more than 20 minutes; (ii) quickly cooling the step (i) in a cooling rate of 70°C/second within a range of Ms+(80°C plus or minus 30°C) and then isothermal heat-treating it for more than 20 minutes; and (iii) oil cooling or air cooling the step (ii).

Description

High strength, high elongation composite steel for bolts with excellent delayed fracture resistance and manufacturing method

The present invention relates to bolt steel used for fastening steel structures and automobile parts, and a method for manufacturing the same, and more particularly, ferrite and bainite, which have high delay fracture resistance and high strength and high elongation, by appropriate control of microstructure. A composite tissue steel and a method for manufacturing the same.

In order to fasten members for efficient construction of steel structures and to reduce weight, multifunction, and high performance of automobile parts, it is necessary to increase the strength of materials used. However, since the high strength of the bolt causes the deterioration of the delayed fracture resistance by hydrogen intrusion, it is currently impossible to use the tensile strength of 130 kg / mm ² or more, and the use of the bolt and its range are limited.

Most of the microstructures in use are quasi single phase tissues of tempered martensite, and carbide-based precipitates are distributed at grain boundaries, and the matrix is deposited in ras martensite. can see. However, the main inhibitory factor in achieving the high strength of the material is a decrease in the delayed fracture resistance due to the intrusion of hydrogen, which is because the precipitate deposited at the grain boundary acts as a trapped site of hydrogen and thus the strength of the grain boundary. It is known to deteriorate. Therefore, the use of high strength bolted steel as a tempered martensite structure has a limitation in terms of microstructure characteristics.

Therefore, in order to achieve high strength of the bolt, it is necessary to first increase the critical delay strength and plastic deformation performance, and for this purpose, improvement of delayed fracture resistance and elongation is inevitable. Therefore, it is desirable to achieve high strength while at the same time pursuing high strength while suppressing the distribution of grain boundary precipitates to the maximum.

The following are the benefits expected when the bolt steel is developed, which has excellent delayed fracture resistance and high strength.

In other words, in terms of steel structures, bolt fastening does not require skilled skills compared to welding joints, and considering the substitution of vulnerable welds. Steel usage can be reduced by reducing the number. In addition, in terms of automotive parts, third, it contributes to the weight reduction of parts, and fourth, there is an advantage that the design diversification and compactness of the vehicle assembly apparatus according to the weight reduction of parts is possible.

Conventional techniques for improving delayed fracture resistance include: 1) corrosion inhibition of steel, 2) minimization of hydrogen intrusion, 3) inhibition of accumulation of diffusible hydrogen contributing to delayed destruction, and 4) steel with high limit diffusion hydrogen concentration. Use, 5) minimization of tensile stress, and 6) relaxation of stress concentration.

As a means to achieve this, it is mainly used to pursue high alloying or to give surface coating or plating to prevent external hydrogen intrusion.

Techniques for manufacturing high strength bainite + martensite composite low carbon alloy steels by heat treatment include Japanese Patent Application Laid-Open No. Hei 6-271975, Hei 7-173531, and Japanese "Iron and Steel Vo.82 (1996) No. 4". have.

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, and is 0.05-0.3% C, 0.1-2.5% Si, 0.1-3.0% Mn, and 0.05-0.1 by weight. 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. Residual austenite is present in a volume fraction of 1-30% in order to secure resistance to delayed destruction by hydrogen in the microstructure. However, the Japanese Patent Laid-Open Publication No. Hei 6-271975 has not achieved a high strength of more than 130kg / mm ² of critical delay fracture strength, and there are many disadvantages in the heat treatment process when manufacturing a composite structure for improving delayed fracture resistance.

Japanese Patent Laid-Open No. 7-173531 has a chemical weight 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 It is a method for producing bainite + martensite abnormal composite tissue steel by continuously cooling steel having a composition after hot forming at a critical cooling rate at which no cornerstone ferrite does not precipitate, but attaining a high strength of 130kg / mm ² or more of critical delay fracture strength. In addition, there are many heat treatment processes in manufacturing a composite structure for improving delayed fracture resistance, which makes it difficult to impart industriality.

The "iron and steel Vol. 82 (1996) No. 4" is based on the conventional tempered martensite structure 0.49% C-0.31% Mn-1.02% Cr-0.68% Mo-0.034% Nb-0.32% V-0.009P-0.004% S is composed of in order to reduce the critical delayed fracture strength of the grain boundary segregation of impurities to 130kg / mm ² grade low P, low S, low Mn screen, for preventing the grain boundary precipitation of carbides Ni, and Cr , Mo, V is added, and V, Nb, and Ti are added for grain refinement, and heat treatment is performed at low tempering temperature. However, the iron and steel Vol. 82 (1996) No. At 4, there is a problem that the delayed fracture resistance is poor to use the critical delay fracture strength of 130kg / mm ² or more.

Accordingly, the present invention relates to a microstructure control method capable of increasing high strength and high elongation without deterioration of elongation at the same time as it is excellent in delayed fracture resistance and high strength in manufacturing high strength bolts of 130 kg / mm ² or more, The objective is to solve the problems of grain embrittlement due to external hydrogen injection, and to provide a composite tissue steel for bolts having high strength and high elongation while having excellent delayed fracture resistance, and to provide a method for manufacturing the same. .

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

The present inventors adjusted the silicon content in an appropriate range in the medium carbon steel, and appropriately heated in an ideal temperature range to control the ferrite phase fraction in the composite structure (ferrite and austenite) in the range of 5-25%, When the austenite in the abnormal zone composite tissue is transformed into bainite tissue by quenching it to the site transformation point immediately after isothermal heat treatment, the improvement effect on delayed fracture resistance is remarkable, and high strength and high elongation can be increased. It is difficult to distribute carbide-based precipitates at the interface, and the distribution of carbide-based precipitates at the bainite and bainite grain boundaries is difficult due to the phase transformation characteristics. I found out that I can manufacture bolted steel .

Starting from the above point of view, the present invention is 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 of the group consisting of the remaining Fe and other unavoidable impurities, the microstructure It has a composite structure of ferrite and bainite, and wherein the phase ratio of the ferrite relates to a high-strength high elongation composite tissue steel with excellent delayed fracture resistance, characterized in that,

In addition, the present invention is a method for producing a steel for bolts, by 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%, A steel composed of one or two or more selected from the group consisting of 0.01-0.2% titanium, 0.01-0.5% tungsten, 0.01-0.2% copper and 0.01-0.5% cobalt, and composed of balance Fe and other unavoidable impurities The ferrite phase fraction in the complex structure of ferrite and austenite is heated by heating for 20 minutes or more within the range of Ac3- (Ac3-Ac1) /1.3, which is an ideal temperature range, from Ac3- (Ac3-Ac1) /5.5. %, Austenitic fraction is controlled to 75-95%, quenched to Ms + (80 ° C ± 30 ° C) range at a cooling rate of 70 ° C / sec or more, and isothermally treated for at least 20 minutes, followed by oil cooling or The present invention relates to a method for producing a high-strength high elongation composite steel for bolts having excellent delayed fracture resistance, characterized by air cooling.

Hereinafter, the chemical component of this invention and the reason for limitation of its range are 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 and elongation as high strength and high elongation bolts for isothermal heat treatment for bainite production. If the content exceeds 0.60%, it is difficult to secure elongation after heat treatment, and it is difficult to obtain bainite structure effective for high elongation due to too high carbon concentration of austenite due to ferrite formation during abnormal reverse heat treatment. This is because it affects decarburization, 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%, it is difficult to secure the strength due to the insufficient effect of strengthening the ferrite in the bainite structure, and also the delayed fracture resistance, elongation, surface corrosion property, impact toughness, bainite structure, and bolting Because it affects permanent denaturation, etc., and it is difficult to properly distribute the surface ferrite decarburization layer in the wire furnace for wire decarburization control, the decarburization is intensified, and it is difficult to control the surface scale characteristics by increasing the hardenability during wire rod cooling. Because there are disadvantages. In addition, if the content exceeds 4.0%, the above-mentioned effect is saturated, which is not preferable because it affects the structure of bainite structure, impact toughness, corrosiveness, fatigue characteristics, etc. in the composite tissue, and is not a blow for wire production. Or because the quality characteristics in the final product are deteriorated due to the inhomogeneity of microstructures due to silicon segregation during billet manufacture, and the thickness of the surface ferrite layer increases during heat treatment, making it difficult to control homogeneous surface decarburization. .

The more preferable component range of the silicon is 2.8-3.3%, which is an isothermal heat treatment time and residual austenite fraction for producing bainite structure, high strength and high elongation of bainite, delayed fracture resistance (diffusive hydrogen amount, grain boundary) Precipitation control), surface decarburization, stress relaxation after bolting or permanent deformation resistance, dynamic and static fatigue characteristics can be improved very effectively.

The manganese (Mn) is an element that forms a solid solution to form a solid solution to strengthen the solid solution and is a very useful element for high-strength bolt characteristics, the content is limited to 0.2-0.8%. When the manganese is added in excess of 0.8%, the solid solution strengthening effect has a more harmful effect on the bolt properties of tissue heterogeneity due to manganese segregation. When the steel solidifies, macro segregation and micro segregation are easy to occur due to the segregation mechanism. Manganese segregation promotes segregation due to the relatively low diffusion coefficient compared to other elements, and the improvement of hardenability results in the core martensite. This is the main reason for generating. In addition, when the manganese is added less than 0.2%, the formation of segregation zone due to manganese segregation is hardly expected, but stress relaxation improvement effect due to solid solution strengthening is difficult to expect.

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 exceeds 0.8%, local quenchability is increased due to manganese segregation and segregation is formed. The deepening of tissue anisotropy, ie, tissue heterogeneity, causes the bolt characteristics 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, the harmful effects of the segregation zone production.

The content of chromium (Cr) is limited to 0.25-0.8%. If the content is less than 0.25%, it is difficult to form a surface ferrite layer for surface decarburization control during heat treatment of high silicon-added steel, so that there is almost no decarburization inhibitory effect and it is difficult to expect an improvement in quenchability. In addition, if the content exceeds 0.8%, it is not preferable because the transformation time of bainite becomes longer during isothermal heat treatment, and it is difficult to generate a surface titration ferrite layer when charging the wire rod for controlling the wire decarburization layer. Because it affects.

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 is less than 0.05%, the distribution of vanadium or niobium-based precipitates in the base material decreases, and thus the role of the non-diffusible hydrogen trap site is insufficient. Therefore, it is difficult to expect the effect of improving the delayed fracture resistance and the stress relaxation is difficult to expect. This is because the improvement effect on resistance is not sufficient, and it is difficult to expect austenite grain refinement, which affects the structure of bainite. When the content exceeds 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 carbides that are not dissolved in the base metal during the austenitic heat treatment increases, thus acting as a nonmetal inclusion. This causes a decrease in fatigue characteristics.

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

The content of nitrogen (N) is limited to 0.005-0.03%. This is because it is difficult to form vanadium and niobium-based nitrides that act as non-diffusion hydrogen trap sites when the content is 0.005% or less, and the effect is saturated when 0.03% or more.

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 with low melting point elements to lower the toughness and forms an emulsion, which has a detrimental effect on delayed fracture resistance and stress relaxation characteristics. It is preferable 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 less than 0.3%, it is difficult to expect the effect of improving the delayed fracture resistance due to the incomplete formation of the surface thickening layer, and the heat treatment time is increased during the graphitization treatment for decarburization control, toughness, and diversification of the manufacturing process. This is because there is no improvement effect of cold forming property, and when the content exceeds 5.0%, the effect is saturated.

The boron (boron, B) is a grain boundary element for improving the hardenability and delayed fracture resistance in the present invention, the content of boron is limited to 0.0010-0.003%. If the content is less than 0.0010%, the grain boundary strength improvement effect due to grain boundary strengthening due to the grain boundary segregation of boron atoms during heat treatment is insufficient, and the graphitization promoting effect is insufficient during the graphitization treatment to improve the cold formability. This is because if the content exceeds 0.003%, the effect is saturated, and the boron-based nitride precipitates at the grain boundary, resulting in a decrease in grain boundary strength.

The content of the molybdenum (Mo) and tungsten (W) is limited to 0.01-0.5%. The reason is that less than 0.01%, it is difficult to obtain the effect of improving stress relaxation by suppressing the growth of cementite when cementite transitions from epsilon carbide to isothermal heat treatment, and finely distributes molybdenum precipitates in isothermal heat treatment. This is because it is difficult to secure stable tissues in the process. If it exceeds 0.5%, the effect is included, and the increase in hardenability makes it easier to form low-temperature tissues (martensite + bainite) during wire rod manufacture, and to graphitize to improve cold formability. This is because there is a disadvantage in that the heat treatment time is long during the treatment.

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 the likelihood of surface flaw is high 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 effect of miniaturizing austenite crystal grains is insufficient, and the precipitation distribution of titanium-based carbonitrides, which is effective for delayed fracture resistance, is insufficient. Therefore, the improvement effect is difficult to be expected. This is because it forms a coarse titanium nitride and 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 a composite tissue steel for bolts using steel having the chemical composition as described above will be described in detail.

In the present invention, the effect is obtained by controlling the ferrite fraction on the complex structure to 5-25%. In other words, the effect of the present invention is to reduce the distribution of precipitates at the grain boundaries, and for this purpose, the microstructure fraction of ferrite must be in the range of 5-25%. For this reason, if the ferrite structure fraction is less than 5%, the amount of ferrite is too small to discontinue the grain boundaries of austenite, and the effect is insufficient. If the content exceeds 25%, the ferrite structure is maintained like the parent material due to the excessive ferrite fraction. This is because it is difficult to obtain a bainite structure effective at high elongation due to a decrease in yield strength and an increase in carbon concentration in austenite due to an increase in ferrite fraction.

More preferred ferrite tissue fraction is in the range of 10-15%. In other words, when the fraction of ferrite tissue in the final composite tissue is 10-15%, the yield ratio (called yield strength / tensile strength ratio) of 0.80 or more is secured, and the distribution of grain boundaries can be effectively controlled and delayed. This is because the effect of improving the fracture resistance can be maximized.

In addition, the ideal reverse heat treatment conditions for securing such a ferrite structure fraction is limited to Ac3- (Ac3-Ac1) /1.3 to Ac3- (Ac3-Ac1) /5.5 temperature range, the reason for the limitation is as follows.

Here, Ac3 is the austenite transformation temperature when heated, Ac1 is the transformation temperature in the abnormal region (ferrite + austenite) when heating, Ac3, Ac1 transformation temperature is different depending on the alloy component system according to the alloy component system.

This is because it is not preferable that the amount of ferrite produced when the reverse heat treatment is higher than 25% at the temperature of Ac3- (Ac3-Ac1) /1.3 is lower than Ac3- (Ac3-Ac1) /, as mentioned above. This is because at 5.5 or higher, the ferrite amount required for grain boundary discontinuity exceeds 5%, so the effect is difficult to expect.

The more ideal heat treatment condition in the present invention is the temperature range of Ac3- (Ac3-Ac1) /1.7 to Ac3- (Ac3-Ac1) /2.5, discontinuity of the grain boundary of austenite, discontinuity of the grain boundary precipitate, plate martensite at the time of hardening The range considers the possibility of formation, heat treatment time and decarburization control.

The heat treatment may be completed by 20 minutes or more to complete the desired transformation, after the heat treatment is quenched at a cooling rate of 70 ℃ / sec or more.

On the other hand, isothermal heat treatment conditions for the production of bainite tissue after heating is limited to the martensite transformation temperature (Ms) directly Ms + (80 ℃ ± 30 ℃).

If the isothermal heat treatment is carried out below Ms + 50 ° C., the bainite transformation time is long and the martensite is likely to occur when an isothermal heat treatment temperature deviation occurs, which is not desirable, and the elongation and impact toughness are reduced. If it exceeds + 110 ℃, there is a problem in securing proper yield strength due to the drastic reduction of yield ratio (yield strength / tensile strength ratio) .Therefore, there is a problem in that stress relaxation is poor when bolting, and fracture resistance is reduced by reducing impact toughness. This is because it is harmful to and also affects the critical delay strength and fatigue characteristics.

The isothermal treatment is used to form a ferrite + bainite composite.

Hereinafter, the present invention will be described in more detail with reference to 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 - - - 0.009 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 and tempering conditions were subjected to a heat treatment test in the heat treatment conditions shown in Table 2 and Table 3 below.

Steel grade used Heating temperature (℃) Ac3-[(Ac3-Ac1) / X] Heating temperature (℃) Heating time (min) Isothermal heating (℃) (Ms + X) Tempering temperature (℃) Isothermal holding time (min) Tempering time (min) Ferrite Percentage (%) Transformation temperature Ac3 Ac1 Ms Inventive Example 7 Invention 2 X = 2 30 X = 80 40 17 955 833 290 Inventive Example 8 Invention 3 X = 2 70 X = 80 40 10 883 803 229 Inventive Example 9 Invention 4 X = 2 80 X = 80 40 8 880 782 260 Inventive Example 10 Invention 5 X = 2 30 X = 80 40 20 961 842 288 Inventive Example 11 Invention 6 X = 2 40 X = 80 40 12 899 817 250 Inventive Example 12 Invention 7 X = 2 120 X = 80 40 8 857 775 208 Comparative Example 9 Comparative Material 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 Comparative Material 4 900 30 Tempering temperature = 450 60 0

Inventive Example 1-6 in Table 2 was prepared so that the ferrite phase fraction in the range of 830-890 ℃ within the range of 818 ℃ to 915 ℃ in the abnormal temperature range of the same alloy component system (Inventive material 1) 5-25% range After quenching at a cooling rate of 70 ° C./sec or more to Ms + (80 ± 30 ° C.), which is an isothermal heat treatment temperature range for bainite transformation, it was prepared by heat treatment for 40 minutes. Where Ms is the martensite transformation start temperature.

In Comparative Example 1-4 in Table 2, the ferrite phase fraction was the same as that of the Inventive Example, in which the ideal reverse heating temperature was Ac3-[(Ac3-Ac1) / (1.3-5.5)] in the same alloy system (Inventive Material 8). After the preparation, it was prepared by heat treatment for 40 minutes of isothermal holding time under conditions of isothermal heating temperature Ms + 110 or more and Ms + 50 ° C or less for bainite production.

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 7-12 was heated at a temperature of Ac3-[(Ac3-Ac1) / 2)], which is the intermediate temperature range of each alloy component of Inventive Materials 2-7, to obtain a ferrite phase fraction of 5-25. Manufactured in% range and quenched at a cooling rate of 70 ℃ / sec or more to Ms + 80 for bainite transformation was maintained after 40 minutes of isothermal holding time and cooled.

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 ℃.

Since Ac3, Ac1, and Ms transformation temperature for determining the microstructure phase fraction, the abnormal range was measured using a thermal analyzer (dilatometry) and the results are shown in Table 2 and Table 3 together.

In order to evaluate the tensile, impact, and delayed fracture characteristics of the materials manufactured as described above, the tensile test piece was used by the KS standard (KS B 0801) No. 4 test piece, and the tensile test was cross head speed. Test at 5 mm / min. The impact test piece was manufactured according to KS standard (KS B 0809) No. 3 test piece, and the notch direction was processed from the side of the rolling direction (LT direction). The microstructure fraction was investigated using a point counting method, which is a general optical microscopy method, wherein the test surface was 1000 mm ² .

In order to show the effect of the present invention, the delayed fracture resistance evaluation was applied to a generally used constant load method. This evaluation method is a general method for evaluating the delayed fracture resistance by the time required for breaking apart under a specific stress or under a specific stress. In the delayed fracture test, the test stress was determined based on the notched tensile strength.

The delayed fracture tester used a constant loading type delayed fracture testing machine. The delayed fracture specimens were prepared with a specimen diameter of 6 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 wolpole buffer solution (HCl + CH ₃ COONa).

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

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 microstructure difference between the invention example and the comparative example is shown in Figure 1, in the invention example 2, the black region is the ferrite structure and the gray region is the bainite structure. The comparative example is a typical tempered martensite tissue. As shown in FIG. 1, the effect of the present invention is to distribute ferrite in the bainite base material in an orderly manner, thereby improving both excellent elongation and delayed fracture resistance.

Tensile Strength (kg / mm ² ) Yield strength (kg / mm ² ) Elongation (%) Cross section reduction rate (%) Impact Toughness (J / cm ² ) Critical Delay Fracture Strength (kg / mm ² ) Inventive Example 1 130 90 20 45 73 130 Inventive Example 2 144 105 33 50 60 140 Inventive Example 3 150 100 35 46 53 150 Inventive Example 4 135 103 25 57 45 135 Inventive Example 5 137 100 21 48 110 135 Inventive Example 6 120 80 30 47 45 120 Comparative Example 1 125 120 16 40 70 110 Comparative Example 2 150 70 18 42 30 110 Comparative Example 3 151 60 16 37 38 100 Comparative Example 4 128 52 17 26 18 95 Comparative Example 5 225 195 6 25 20 100 Comparative Example 6 220 190 8 30 25 110 Comparative Example 7 180 165 10 35 40 115 Comparative Example 8 145 130 11 40 30 120

Tensile Strength (kg / mm ² ) Yield strength (kg / mm ² ) Elongation (%) Cross section reduction rate (%) Impact Toughness (J / cm ² ) Critical Delay Fracture Strength (kg / mm ² ) Inventive Example 7 151 120 33 55 60 150 Inventive Example 8 164 128 28 60 65 150 Inventive Example 9 155 120 33 48 56 150 Inventive Example 10 153 123 30 49 50 150 Inventive Example 11 158 124 32 54 56 150 Inventive Example 12 166 129 29 47 50 150 Comparative Example 9 147 135 15 57 30 130 Comparative Example 10 147 129 16 58 20 110 Comparative Example 11 148 139 15 57 40 140 Comparative Example 12 110 95 15 60 50 100

As shown in Table 4 and Table 5, the elongation of the inventive examples is 21-35% and the critical delay fracture strength is 120-150kg / mm2, but the comparative examples are 8-18% and 95-140kg / mm2, ferrite. The invention using the composite structure consisting of + bainite can be seen that the critical delay strength is significantly improved compared to the comparative example. Here, the ferrite tissue fraction to show the effect of the present invention was able to most effectively improve the critical delay strength in the 5-25% range.

As described above, the present invention proposes an alloy composition system and heat treatment conditions of the composite structure steel composed of ferrite + bainite to improve the excellent elongation and delayed fracture resistance, thereby attaining high strength of the bolts and at the same time ensuring excellent delayed fracture resistance. As it is possible to provide a high-strength, high elongation composite tissue steel for bolts.

Claims (6)

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- 0.2%, cobalt 0.01-0.5%, optionally containing one or two or more of the group consisting of the remaining Fe and other unavoidable impurities, the microstructure has a complex structure of ferrite and bainite, wherein High-strength, high elongation composite steel for bolts with excellent delayed fracture resistance, characterized in that the phase fraction of ferrite is 5-25% In the method for producing 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 0.01-0.5%, titanium 0.01-0.2% , Steel containing 0.01 to 0.5% of tungsten, 0.01 to 0.2% of copper, 0.01 to 0.5% of cobalt and optionally containing one or two or more of the remaining Fe and other unavoidable impurities. Heated for at least 20 minutes in the range of Ac1) /1.3 to Ac3- (Ac3-Ac1) /5.5, the ferrite phase fraction is 5-25% and the austenite fraction is 75-95% in the composite structure of ferrite and austenite. Delayed, characterized in that the control is quenched and cooled to Ms + (80 ° C. ± 30 ° C.) at a cooling rate of 70 ° C./sec or more, and then isothermally treated for at least 20 minutes, followed by oil cooling or air cooling. How high strength and excellent resistance to the masses manufacturing a composite structure steel for the bolt elongation The method of claim 2, The silicon is a method of manufacturing a high-strength high elongation composite tissue steel for bolts with excellent delayed fracture resistance, characterized in that contained in the range of 2.8-3.3% The method of claim 2, The high strength and high elongation of the composite tissue steel for bolts, characterized in that the heating of the steel is heated for more than 20 minutes within the range of Ac3- (Ac3-Ac1) /1.7 to Ac3- (Ac3-Ac1) /2.5. Manufacturing method The method of claim 2, The ferrite phase fraction is 10-15%, characterized in that the method of manufacturing a high-strength high elongation composite tissue steel for bolts excellent in delayed fracture resistance The method of claim 2, The isothermal heat treatment is performed in the range of Ms + (80 ° C. ± 15 ° C.).