KR100376532B1 - High strength duplex steel with a good delayed fracture resistance and a method of manufacturing therefor - Google Patents

Abstract

The present invention relates to a composite structure steel for high strength bolts having excellent delayed fracture resistance and a method for manufacturing the same, and its purpose is to produce high strength bolts of 130 kg / mm ² or more, and has excellent delayed fracture resistance and high-strength microstructure. The purpose is to provide a control plan.

The present invention for achieving the above object 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%, less than or equal to 0.005% oxygen, 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%, It optionally contains one or two or more of the group consisting of 0.01-0.5% tungsten, 0.01-0.2% copper and 0.01-0.5% cobalt, and is composed of balance Fe and other unavoidable impurities, and its microstructure is ferrite and temper The present invention relates to a composite steel for high-strength bolts having a composite structure of de-martensitic, wherein the phase ratio of ferrite is 8-20%, and excellent in delayed fracture resistance.

In addition, the present invention is a method for producing a steel for bolts, heating the steel of the same component as described above Ac3- (Ac3-Ac1) / 1.3 to Ac3- (Ac3-Ac1) / 5.5 or more for 20 minutes In the composite structure of ferrite and austenite, the ferrite phase fraction is controlled to 8-20% and the austenite fraction is set to 80-92%, and it is quenched at a cooling rate of 70 ° C / sec or more and over 20 minutes in the range of 300-600 ° C. The present invention relates to a method for manufacturing a high strength bolted composite steel having excellent delayed fracture resistance, characterized by tempering.

Description

STRENGTH DUPLEX STEEL WITH A GOOD DELAYED FRACTURE RESISTANCE AND A METHOD OF MANUFACTURING THEREFOR}

The present invention relates to bolt steel used for fastening steel structures and automobile parts, and a method of manufacturing the same, and more particularly, ferrite and tempered martensite composites having high delay fracture resistance and high strength through appropriate control of microstructures. It relates to a tissue steel and a method of 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 the high strength of the bolt, it is inevitable to improve the delayed fracture resistance to increase the critical delay fracture strength, and as a means to achieve the high strength while suppressing the distribution of the grain boundary precipitate 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 in consideration of substituting weak welds, firstly, stability of steel structures can be increased by tightening force when bolting, and secondly, bolt fastening. 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.

The Japanese Laid-Open Patent Publication No. Hei 6-271975 relates to a method for manufacturing a composite steel with excellent resistance to delayed fracture by hydrogen, wherein the weight is 0.05-0.3% C, 0.1-2.5% Si, 0.1-3.0% Mn, 0.05- In steels containing at least one alloy element among 0.1Al, Cu, Ni, Mo, Cr, Nb, V, Ti, and B, the microstructure is a martensite single phase, bainite single phase, or bainite + martensite composite structure. In order to secure the delayed fracture resistance by hydrogen in such a microstructure, residual austenite is present in a volume fraction of 1-30%. 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 high strength and excellent delayed fracture resistance in manufacturing high-strength bolts of 130 kg / mm ² or more, and its purpose is to have a fine composite structure effective for delayed fracture resistance. Firstly, due to the discontinuity of the austenite grain boundary due to the homogeneous distribution of ferrite, the problem of grain embrittlement due to external hydrogen infiltration at tempered martensite grain boundary can be significantly reduced. Due to the nature of crack propagation, the crack propagation speed can be reduced by the effect of blunting of the crack tip. Third, it induces the change of grain boundary characteristics between tempered martensite and precipitates of grain boundary between ferrite and tempered martensite. Indirect grain boundary strengthening effect by reducing hydrogen trap site due to suppression of distribution And to provide for a composite organization can expect the river,

In addition, the purpose is to construct a complex structure of ferrite and tempered martensite through the inverse heat treatment of the distribution of grain boundary precipitates inevitably precipitated in the existing tempered martensite steel, thereby improving delayed fracture resistance and high strength. An object of the present invention is to provide a method of manufacturing bolt steel that can be realized.

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

The present inventors adjust the silicon content in an appropriate range in the medium carbon steel, and appropriately heated within the ideal temperature range, to control the ferrite phase fraction in the composite structure (ferrite and austenite) in the range of 8-20% to delay fracture resistance In the case of constructing a microcomposite structure effective in the process of quenching and then manufacturing it through a hardening process, the precipitation frequency of grain boundary precipitates, which is harmful to the delayed fracture resistance, is reduced compared to the tempered martensite structure, and thus the excellent delayed fracture resistance even at a tensile strength of 130kg / mm ² or more. It was found out that.

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 temper martensite, wherein the phase ratio of the ferrite relates to a composite steel for high-strength bolts excellent in 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%, 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 which optionally contains one or two or more of 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 is composed of balance Fe and other unavoidable impurities. Heated at least 20 minutes within the range of Ac3- (Ac3-Ac1) /1.3, which is the ideal temperature range, from Ac3- (Ac3-Ac1) /5.5. %, Austenitic fraction is controlled to 80-92%, quenched at a cooling rate of 70 ℃ / sec or more, and delayed destruction characterized in that tempering more than 20 minutes in the 300-600 ℃ range Resistance is a method for producing a composite structure steel for superior high-strength bolts.

Hereinafter, the chemical component of the present invention and the range thereof will be described.

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 yield strength as a high-strength bolt steel by hardening and annealing after abnormal reverse heat treatment. , When the content exceeds 0.60%, plate martensite when hardened due to difficulty in securing toughness due to high strength, and annealing due to an increase in carbon concentration of austenite due to ferrite formation during an abnormal reverse heat treatment to show the effect of the present invention. This is because quench cracking is likely to occur due to the formation of.

The content of silicon (Si) is limited to 2.0-4.0% because of the following reasons. In other words, when the silicon content is less than 2.0%, Ac1 transformation point is low and the time required for diffusion transformation to obtain a composite structure is long.There is a difficulty in securing strength due to insufficient softening resistance effect when tempering. It is difficult to suppress precipitation of grain boundary precipitates to improve the resistance, and it has an effect on the increase of the amount of diffusible hydrogen which has a detrimental effect on delayed fracture, delayed fracture resistance, surface corrosion characteristics, impact toughness, composite structure and permanent deformation during bolting. Because it's crazy. In addition, it is difficult to properly distribute the surface ferrite decarburization layer in the wire heating furnace for controlling the decarburization of the wire, which leads to intensification of decarburization, and it is difficult to control the surface scale characteristics by increasing the hardenability during wire rod cooling. This is because it is difficult to control the initial tissue before the abnormal heat treatment to secure the composite tissue. On the other hand, when the silicon content exceeds 4.0%, the above-mentioned effect is not preferable because it saturates and affects quenchability, composite structure, impact toughness, fatigue characteristics, and the like. Or because the quality characteristics in the final product are deteriorated due to the inhomogeneity of the microstructure due to the segregation of silicon during the manufacture of the billet. Because.

The more preferable component range of the silicon is 2.8-3.3%, which is an ideal reverse heat treatment time and composite tissue fraction control for producing a composite tissue, high strength of the base material (ferrite and tempered martensite), temper brittleness, delayed fracture resistance ( This is because diffusion hydrogen content, precipitation control of grain boundary precipitates, surface decarburization, stress relaxation or permanent strain resistance after bolting, and dynamic and static fatigue characteristics can be improved 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 tend to occur depending on the segregation mechanism, and manganese segregation promotes segregation due to the relatively low diffusion coefficient compared to other elements. 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%, epsilon carbide becomes considerably unstable during tempering, and the transition to cementite rapidly progresses, which inhibits stabilization of microepsilon carbides effective for stress relaxation and promotes cementitization. It is not desirable to improve the stress relaxation by reducing the softening resistance of the base material, and it is not preferable because the transformation time is longer during abnormal reverse heat treatment for producing the composite structure. This is because it is difficult to generate the surface titration ferrite layer at the time of entrance, which affects the homogeneous decarburization control.

The vanadium (V) or niobium (Nb) is an element that improves delayed fracture resistance and stress relaxation resistance, and its content is limited to 0.05-0.2%. If the content is less than 0.05%, the distribution of vanadium or niobium-based precipitates in the base material decreases, and thus the role as a non-diffusible hydrogen trap site is insufficient. This is because the improvement effect on the resistance is not sufficient. If 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 coarse alloy which is not dissolved in the base metal during the austenitic heat treatment. Since the amount of carbide increases and acts like a non-metallic inclusion, the fatigue property is reduced.

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 if the content exceeds 5.0%, the effect is saturated, and the amount of retained austenite at the time of quenching causes temper embrittlement during tempering, leading to deterioration of impact toughness.

The content of oxygen (O) is limited to 0.005% or less. If the content is more than 0.005%, 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, respectively. 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 boron (boron, B) is limited to the content of boron in the grain boundary strengthening element for improving the hardenability and delayed fracture resistance in the present invention to 0.0010-0.003%. If the content is less than 0.0010%, the effect of improving the grain boundary strength 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 precipitation of boron nitride at the grain boundary lowers the grain 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% of cementite inhibits the growth of cementite when cementite transitions from epsilon carbide during tempering, and it is difficult to obtain the effect of improving stress relaxation.It is stable at high temperature by finely distributing molybdenum precipitates during tempering. This is because it is difficult to secure the tissue, and when it exceeds 0.5%, the effect is included, and the hardenability increases the formation of low-temperature tissues (martensite + bainite) during wire rod manufacturing, and during the graphitization treatment to improve cold formability. This is because the heat treatment time is long.

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 achieved by controlling the ferrite fraction on the composite tissue to 8-20%. In other words, the effect of the present invention is to reduce the distribution of precipitates in the grain boundary, and for this purpose, the microstructure fraction of ferrite must be in the range of 8-20%. For this reason, if the ferrite structure fraction is less than 8%, the amount of ferrite is too small to discontinue the grain boundary of austenite, and the effect is insufficient. If the content exceeds 20%, the ferrite structure is maintained like the parent material due to the excessive ferrite fraction. This is because the decrease in yield strength and the increase in the carbon concentration in austenite due to the increase of the ferrite fraction can lead to the formation of plate martensite.

More preferred ferrite tissue fraction is in the range of 10-15%. That is, 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 can be secured, and the distribution of grain boundaries can be effectively controlled and delayed destruction is achieved. This is because the improvement effect of 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 heat treatment time is 20 minutes or more, 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 at the time of the abnormal reverse heat treatment above Ac3- (Ac3-Ac1) /1.3 is higher than 20%, which leads to a decrease in yield strength as mentioned above, and Ac3- (Ac3-Ac1) / Above 5.5, the effect is not expected because the amount of ferrite required for grain boundary discontinuity exceeds 8%.

More preferred heat treatment conditions in the present invention is a temperature range of Ac3- (Ac3-Ac1) /1.5 to Ac3- (Ac3-Ac1) /4.0, discontinuity of the grain boundary of the austenite grains, discontinuity of the grain boundary precipitates, plate martensite upon quenching The range considers the possibility, 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, in the present invention, the tempering temperature is limited to the range of less than 1 hour at 300-600 ° C because elongation and impact toughness are not preferable at the tempering temperature below 300 ° C, and it is difficult to secure the strength above 600 ° C. . This is because if the tempering time exceeds 1 hour, the heat treatment effect is saturated.

More preferred tempering temperature range in the present invention is 450-550 ℃ tensile strength, yield strength, elongation, cross-sectional reduction rate, delayed fracture resistance, impact toughness, distribution of precipitates of hydrogen trap site, stress relaxation after bolting (permanent deformation resistance) ) And the amount of retained austenite.

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

Example

Steels having a component composition as shown in Table 1 were prepared, and the inventive material (1-8) satisfies the component range of the present invention, and the comparative material (1-4) is those outside the component range of the present invention. The prepared steels were cast into 50 kg ingot, homogenized and 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 Oxygen content of the steel is 0.003 ± 0.001% by weight

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 time (min) Tempering temperature Ferrite Percentage (%) Transformation temperature Temperature (℃) Time (min) Ac3 Ac1 Inventive Example 1 Invention 1 X = 4.0 60 500 40 20 915 818 Inventive Example 2 Invention 1 X = 2.0 40 500 40 12 Inventive Example 3 Invention 1 X = 1.6 30 500 40 8 Comparative Example 1 Invention Material 8 X = 6.0 120 500 40 25 915 818 Comparative Example 2 Invention Material 8 X = 8.0 40 500 40 28 Comparative Example 3 Invention Material 8 X = 1.3 30 500 40 6 Comparative Example 4 Invention Material 8 Heating temperature = 950 30 300 60 0 - - Comparative Example 5 Invention Material 8 Heating temperature = 950 30 400 60 0 Comparative Example 6 Invention Material 8 Heating temperature = 950 30 500 60 0 Comparative Example 7 Invention Material 8 Heating temperature = 950 30 630 60 0

Steel grade used Heating temperature (C) Ac3- [Ac3-Ac1) /%] Heating time (min) Tempering temperature Ferrite Percentage (%) Transformation temperature Temperature (℃) Time (min) Ac3 Ac1 Inventive Example 4 Invention 2 894 30 500 40 17 955 833 Inventive Example 5 Invention 3 843 70 500 40 11 883 803 Inventive Example 6 Invention 4 831 80 500 40 9 880 782 Inventive Example 7 Invention 5 902 30 500 40 20 961 842 Inventive Example 8 Invention 6 858 40 500 40 13 899 817 Inventive Example 9 Invention 7 816 120 500 40 8 857 775 Comparative Example 8 Comparative Material 1 900 30 450 60 0 - - Comparative Example 9 Comparative Material 2 900 30 450 60 0 Comparative Example 10 Comparative Material 3 950 30 450 60 0 Comparative Example 11 Comparative Material 4 900 30 450 60 0

Inventive Example 1, Inventive Example 2, and Inventive Example 3 in Table 2 are ferrite within the range of Ac3-[(Ac3-Ac1) / (1.6-4.0)] The phase fraction was prepared in the range of 8-20%, and Comparative Example 1, Comparative Example 2, and Comparative Example 3 had an abnormal reverse heating temperature of Ac3-[(Ac3-Ac1) / (1.5 each) in the same alloy system (Inventive Material 8). , 6.0, 8.0)]. On the other hand, Comparative Example 4, Comparative Example 5, Comparative Example 6, Comparative Example 7 was prepared in the conventional tempered martensite structure having a ferrite phase percentage of 0% in the same alloy component system (Inventive Material 8).

Table 3 shows the ferrite phase fraction in the range of 8-20% by heating at the temperature of Ac3-[(Ac3-Ac1) / 2)], which is the intermediate temperature range of the alloying components of the invention 2-7. After cooling, tempering was performed at 500 ° C. On the other hand, Comparative Example 1-4 was heated in the 900-950 ℃ range of austenite single phase region, oil-cooled and tempered at 450 ℃.

Since Ac3 and Ac1 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.

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 a ferrite structure and the gray region is a tempered martensite structure. The comparative example is a typical tempered martensite tissue. As shown in FIG. 1, the effect of the present invention is to improve the delayed fracture resistance by randomly distributing ferrite in the tempered martensite base material.

Tensile Strength (kg / mm ² ) Yield strength (kg / mm ² ) Elongation (%) Cross section reduction rate (%) Impact Toughness (J / cm ² ) Critical Delay Break Strength (kg / mm ² ) Inventive Example 1 145 120 15 44 42 150 Inventive Example 2 155 134 15 45 40 150 Inventive Example 3 175 153 12 40 33 150 Comparative Example 1 122 91 11 40 12 110 Comparative Example 2 121 82 12 40 10 110 Comparative Example 3 176 155 8 31 37 120 Comparative Example 4 225 195 4 25 20 - Comparative Example 5 220 190 6 30 25 Comparative Example 6 180 165 8 35 40 Comparative Example 7 125 115 14 55 57 120

Tensile Strength (kg / mm ² ) Yield strength (kg / mm ² ) Elongation (%) Cross section reduction rate (%) Impact Toughness (J / cm ² ) Critical Delay Break Strength (kg / mm ² ) Inventive Example 4 172 146 14 44 45 150 Inventive Example 5 183 160 16 49 51 150 Inventive Example 6 178 156 14 37 45 150 Inventive Example 7 173 137 14 39 37 150 Inventive Example 8 175 147 14 42 42 150 Inventive Example 9 184 164 15 35 37 150 Comparative Example 8 147 135 15 57 30 130 Comparative Example 9 147 129 16 58 20 110 Comparative Example 10 148 139 15 57 40 140 Comparative Example 11 110 95 15 60 50 100

Table 4 and the critical delayed fracture strength of the Inventive Example, as shown in Table 5 is 150kg / mm ² level or the comparative examples are 100-140kg / mm ² level, to using a ferrite composite structure honor contrast threshold delay of Comparative Example It can be seen that the fracture strength is significantly improved. Here, the ferrite tissue fraction for showing the effect of the present invention was able to most effectively improve the critical delay strength in the range of 8-20%.

As described above, the present invention proposes an alloy composition system and heat treatment conditions of the ferritic composite steel for improving the delayed fracture resistance, thereby achieving a high strength of the bolt and at the same time ensures excellent delayed fracture resistance composites for high strength bolts It can provide a tissue lecture.

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 temper martensite At this time, the phase ratio of the ferrite is 8-20% composite steel for high strength bolts excellent in delayed fracture resistance In the method of manufacturing the steel for bolts, By weight%, contains carbon 0.4-0.60%, silicon 2.0-4.0%, manganese 0.2-0.8%, chromium 0.25-0.8%, phosphorus 0.01% or less, sulfur 0.01% or less, nitrogen 0.005-0.01%, oxygen 0.005% or less In addition, vanadium 0.05-0.2%, niobium 0.05-0.2%, nickel 0.3-2.0%, boron 0.001-0.003%, molybdenum 0.01-0.5%, titanium 0.01-0.2%, tungsten 0.01-0.5%, copper 0.01- Steels which optionally contain one or two or more of the group consisting of 0.2%, 0.01-0.5% of cobalt, and are composed of residual Fe and other unavoidable impurities, are obtained from Ac3- (Ac3-Ac1) /1.3 to Ac3- (Ac3- Heating at least 20 minutes within the range of Ac1) /5.5, controlling the ferrite phase fraction to 8-20% and the austenite fraction to 80-92% in the composite structure of ferrite and austenite, and cooling at 70 ° C / sec or more. Method for producing a high strength bolted composite steel with excellent delayed fracture resistance, characterized by rapid cooling at a speed and tempering in the range of 300-600 ° C. for at least 20 minutes. The method of claim 2, The silicon is a method for producing a high-strength bolted composite steel with excellent delayed fracture resistance, characterized in that it is contained in the range of 2.8-3.3% The method of claim 2, The method of manufacturing a high strength bolted composite steel with excellent delayed fracture resistance, characterized in that the heating of the steel is carried out for more than 20 minutes in the range of Ac3- (Ac3-Ac1) /1.5 to Ac3- (Ac3-Ac1) /4.0. The method of claim 2, The ferrite phase fraction is 10-15%, characterized in that the manufacturing method of the composite steel for high strength bolts excellent in delayed fracture resistance The method of claim 2, The tempering temperature is 450-550 ℃ characterized in that the manufacturing method of the high-strength bolted composite steel having excellent delayed fracture resistance