KR20010060701A - High strength steel having a superior delayed fracture resistance and bolt made of the steel and method for manufacturing working product by using it - Google Patents

Abstract

PURPOSE: A process for preparing a fine composite structure steel having excellent delayed fracture resistance and capable of being used in a delayed fracture strength of 150kg/mm¬2 or more by basically solving hydrogen embrittlement and process for production of steel articles using the same are provided. Therefore, the obtained steel can be used as a high strength bolt. CONSTITUTION: The subject high strength steel comprises by weight, C:0.65 to 1.50%, Si:2.0 to 4.0%, Mn:0.1 to 0.8%, P and S:0.01% or less, N:0.005 to 0.01%, O:0.05% or less, one or two or more selected from Ni:0.3 to 2.0%, B:0.001 to 0.003%, V:0.01 to 0.5%, Nb:0.01 to 0.5%, Mo:0.01 to 0.5%, Ti:0.01 to 0.2%, W:0.01 to 0.5% and Cu:0.01 to 0.2% and the balance of Fe with other inevitable impurities. The fine structure of the steel is a composite structure of ferrite and retained austenite having a phase fraction of 20 to 35%.

Description

High strength steel having a superior delayed fracture resistance and bolt made of the steel and method for manufacturing working product by using it}

The present invention relates to a steel requiring high strength and excellent delayed fracture resistance. More specifically, the micro composite structure of ferrite and residual austenite is used to control the phase fraction of the brachy type in the residual austenite to significantly improve other mechanical properties. It relates to a high strength steel, a bolt having the composition of the steel, and a method for producing a steel workpiece from the steel.

In general, high-strength alloys are characterized by the fact that when a constant load is continuously applied, hydrogen diffuses into a specific area within the material and cracks develop, which is called delayed fracture. Steel workpieces with continuous loads with high strength alloys are evaluated as important properties of delayed fracture resistance, and the representative example is bolts.

The bolt is a member for efficient construction of steel structures, and the strength of the material must be increased for light weight, multifunction, and high performance of automobile parts. However, the high strength of the bolt material is a problem that the delayed fracture resistance due to hydrogen infiltration is deteriorated, it is currently impossible to use more than 130kg / mm ² grade tensile strength, the use and range of the use is limited.

The followings are the benefits of developing bolt steel with excellent delayed fracture resistance and high strength. That is, in terms of steel structure, bolt fastening does not require skilled skills compared to welding joint, and considering the advantages of replacing weak welds, first, it is possible to increase the stability of steel structures by strengthening bolt fastening force, and second, the number of bolt fastenings. By reducing the amount of steel used can be reduced. In addition, in terms of automobile parts, third, it contributes to the lightening of parts. Fourth, there is an advantage that the design diversification and compactness of the automobile assembly apparatus according to the lighter parts are possible. Therefore, if the high strength can be achieved without deteriorating the resistance of the delayed fracture resistance of the material, it is expected that the ripple will be considerably large considering the advantages in use and the effect on the industry.

Conventional bolt materials are mostly microstructured quasi single phase of tempered martensite, in which Fe-based precipitates are distributed at grain boundaries and precipitates are also distributed in the base material of lath martensite. This is a general feature. However, it is known that the precipitates distributed at the grain boundaries thus act as trapped sites of hydrogen and degrade the strength of the grain boundaries, resulting in a decrease in delayed fracture resistance and inability to achieve high strength of the material. Therefore, it is accepted that the microstructure of tempered martensite has a limit in achieving high strength of steel for bolts.

Therefore, in order to achieve high strength without the resistance of the delayed fracture resistance of the bolt, it can be seen that it is most important to suppress the distribution of the Fe-based precipitates distributed at the grain boundaries after the heat treatment to the maximum. Conventional techniques for improving delayed fracture resistance are exemplified in Japanese Patent Application Laid-Open Nos. 6-271975, 7-173531, and Japanese "Steel Steel Vol. 82 (1996) No. 4".

The Japanese Laid-Open Patent Publication No. 6-271975 relates to a method for manufacturing a composite steel structure having excellent resistance to delayed fracture by hydrogen. The weight is 0.05-0.3% C, 0.1-2.5% Si, 0.1-3.0% Mn, 0.05 -0.1% Al, including Cu, Ni, Mo, Cr, Nb, V, Ti, B, or one or more alloying elements, and its microstructure is martensite and bainite single phase or composite structure, and this microstructure In order to ensure resistance to delayed destruction by hydrogen, residual austenite is present in a volume fraction of 1-30%. However, in the prior art, it is not possible to achieve a high strength of 130 kg / mm ² or more in the critical delayed fracture strength, and also, a heat treatment process for obtaining a composite structure for improving the delayed fracture resistance has many disadvantages.

Japanese Patent Laid-Open No. 7-173531 discloses 0.05-0.3% C, 0.05-2.0% Si, 0.3-5.0% Mn, 1.0-3.0% Cr, 0.01-0.5% Nb, and 0.01-0.06% Al by weight. The present invention relates to a method for producing bainite + martensite abnormal composite tissue steel by continuously cooling a steel having a chemical composition of not more than a critical cooling rate at which cornerstone ferrite does not precipitate, and also a critical delay fracture strength of 130kg / mm. ^It has not achieved higher strength than ² , and there are too many heat treatment processes to obtain a composite structure for improving the delayed fracture resistance, so the industrial use value is low.

The Japanese "Iron and Steel Vol. 82 (1996) No. 4" is 0.49% C-0.31% Mn-1.02% Cr-0.68% Mo-0.034% Nb-0.32% based on the conventional tempered martensite structure. is composed of an alloy component of the V-0.009% P-0.004% S, the critical delayed fracture strength Chemistry low P, low S, low Mn for grain boundary segregation, the reduction of impurities to 130kg / mm ² grade, to prevent the grain boundary precipitation of carbides For Ni, Cr, Mo, V is added, and V, Nb, Ti is added for grain refinement, characterized in that the heat treatment at a low tempering temperature. However, there is a problem in that the delay fracture resistance is poor to use the material provided in the prior art with a critical delay fracture strength of 130 kg / mm ² or more.

Accordingly, the present inventors have filed in Korean Patent Application Nos. 98-50898, 98-50899, and 98-50900 for bolt steels having improved delayed fracture resistance to 150 kg / mm ² level.

The Korean Patent Application No. 98-50898 is weight percent C: 0.4-0.60%, Si: 2.0-4.0%, Mn: 0.2-0.8%, Cr: 0.25-0.8%, V, Nb, Ni, B, Mo Austenitic single phase was heated by heating at least one steel, Ti, W, Cu, and Co at least 20 minutes at a temperature of Ac ₃ transformation point, and then austenite single phase was obtained at a cooling rate of 70 ° C./sec or higher. The present invention relates to a method of manufacturing a bolt steel having a critical delay fracture strength of 150 kg / mm ² by quenching to + 60 ° C. and then incubating for at least 20 minutes and allowing the microstructure to have bainite.

In addition, the Korean Patent Application No. 98-50899 discloses that the steel of the component system proposed in the application number 98-50898 is Ac _3- (Ac ₃ -Ac ₁ ) /1.3 in the ideal temperature range Ac _3- (Ac _3- By heating at least 20 minutes in the range of Ac ₁ ) /5.5, a composite structure having a ferrite phase fraction of 5-25% and an austenite phase ratio of 75-95% is obtained, followed by Ms at a cooling rate of 70 ° C / sec or more. After quenching to the range of + 50 ℃ ~ Ms + 110 ℃ and incubating for more than 20 minutes, the microstructure is a composite structure of ferrite and bainite, and the ferrite phase fraction has 5-25%. It relates to a method of manufacturing bolt steel of mm ² level.

In addition, the Republic of Korea Patent Application No. 98-50900 mislead the Application No. 98-50898 in the steel over the range of the inverse temperature-component system proposed in the _{Ac 3 - (Ac 3 -Ac 1} ) Ac 3 in /1.3 - (Ac ₃ -Ac ₁ ) / 5.5 minutes by heating for more than 20 minutes to obtain a composite structure having a ferrite phase fraction of 8-20% and austenite fraction of 80-92%, which was quenched at a cooling rate of 70 ℃ / sec or more 300 The present invention relates to a method of manufacturing bolt steel with a critical delay fracture strength of 150 kg / mm ² by tempering at -600 ° C. for 20 minutes or more so that the microstructure has a composite structure of ferrite and tempered martensite.

The above-mentioned Korean Patent Application Nos. 98-50898, 98-50899, and 98-50900 have the meaning of improving the critical delay fracture strength of the steel for bolts by 150 kg / mm ² , but can give meaning to them. There is a limit that does not improve the fracture strength to more than 150kg / mm ² level.

The present invention was completed in a series of studies to improve the high-strength and critical delay fracture strength to more than 150kg / mm ² level by overcoming the technical limitations of the prior art described above, the object is the intergranular embrittlement according to external hydrogen intrusion By fundamentally solving the problem of the present invention provides a steel having a microcomposite structure having excellent delayed fracture resistance, a method for manufacturing a workpiece, and a bolt obtained therefrom.

Steel of the present invention for achieving the above object, by weight% carbon 0.65-1.50%, silicon 2.0-4.0%, manganese 0.1-0.8%, chromium 0.1-0.8%, phosphorus and sulfur 0.01% or less, nitrogen 0.005-0.01 %, 0.005% or less of oxygen, including nickel 0.3-2.0%, boron 0.001-0.003%, vanadium: 0.01-0.5%, niobium: 0.01-0.5%, 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 and 0.01-0.2% copper, and is composed of the remaining Fe and other impurities.

In addition, the manufacturing method of the steel workpiece of the present invention, carbon by weight 0.65-1.50%, silicon 2.0-4.0%, manganese 0.1-0.8%, chromium 0.1-0.8%, phosphorus 0.01% or less, sulfur 0.01% or less, nitrogen 0.005-0.01%, oxygen 0.005% or less, including nickel 0.3-2.0%, boron 0.001-0.003%, vanadium: 0.01-0.5%, niobium: 0.01-0.5%, molybdenum 0.01-0.5%, titanium 0.01- Austenitic steels containing one or two or more selected from the group consisting of 0.2%, 0.01-0.5% tungsten and 0.01-0.2% copper and composed of the remaining Fe and other impurities are heated at a temperature above the Ac ₃ transformation point. It is a single phase, quenched to a temperature of Ms + 50 ° C to Ms + 215 ° C at a cooling rate of 20 ° C / sec or more, isothermally treated for at least 60 minutes, and cooled or air-cooled to form a composite structure of residual austenite and ferrite having a percentage of 15% or more It is configured to include a.

Hereinafter, the present invention will be described in detail.

In the present invention, the term 'steel workpiece' refers to a workpiece (molded product) processed in a certain form of steel, and includes all workpieces that can use the properties of delayed fracture resistance and high strength of the steel component system of the present invention. For example, the steel of the present invention is a component system useful for producing as a wire rod, and there are steel products such as bolts and springs that can be processed into the wire rod.

The present inventors fundamentally suppress the precipitation of grain boundary precipitates (Fe-based), which are harmful to delayed fracture resistance at the austenite grain boundary in the microstructure aspect, while the microstructure capable of the critical delay fracture strength of 150 kg / mm ² or more in terms of tensile strength. As a result of multi-degree study on the control method, high carbon steel with carbon content of 0.65 ~ 1.5% is controlled to 2.0 ~ 4.0% of silicon content, and it is quenched directly to Martensite transformation point (Ms) after heating above Ac ₃ transformation point during heat treatment. After isothermal (constant temperature) heat treatment to produce a fine composite structure (ferrite + residual austenite),

1) ferrite and retained austenite have a lamellar distribution similar to that of pearlite, and the spacing between these phases can be significantly fined in the range of about 0.1 to 0.3 μm to achieve even higher strength,

2) It is possible to fundamentally solve the problem of precipitate-related grain embrittlement due to external hydrogen infiltration, because there are no precipitates anywhere in the grain boundary or grains due to the microstructure characteristics, and

3) If the phase fraction of the retained austenite in the composite structure is within the range of 20-75%, it is possible to manufacture bolted steel with a critical delay strength of 150kg / mm ² or more, which is excellent in delayed fracture resistance and attains further strengthening. Figured out. The bolt steel and the manufacturing method of the present invention completed in this respect will be described separately.

[High strength steel with excellent delayed fracture resistance]

It is preferable to make content of carbon (C) into 0.65-1.5%. If the carbon content is less than 0.65%, it is difficult to obtain an adequate amount of retained austenite, shape and size in the ferritic + residual austenite after heat treatment for the production of ferritic + residual austenite composite steel. This is because it is difficult to secure sufficient tensile strength and yield strength as molten steel. In addition, when the carbon content is more than 1.50%, characteristics such as cross-sectional reduction rate, elongation rate and impact toughness decrease after heat treatment, segregation and surface flaws occur during wire rod manufacturing, and surface decarburization deepens when charging furnace, and bolts are fastened. Permanent deformation and static fatigue characteristics deteriorate, the appropriate shape and size of microcomposite tissues, the transformation time required to secure ferrite + residual austenite complex tissues, carbon concentration in residual austenite, and interfacial tool distribution Because it has a bad effect. The preferred carbon content in the present invention is 0.8 to 1.0%, which is more advantageous for the isothermal heat treatment time for producing the ferrite + residual austenite composite structure and the fraction, size and shape of the appropriate residual austenite. In addition, it is possible to improve the strength of the composite structure, high toughness, high elongation, high section reduction rate, delay fracture resistance (diffuse hydrogen amount, precipitation control of grain boundary precipitate), suppress surface decarburization during heat treatment, and bolt fastening This is because post stress relaxation or permanent strain resistance, dynamic and static fatigue characteristics can be improved very effectively.

It is preferable to limit the content of silicon (Si) to 2.0-4.0%. If the silicon is less than 2.0%, the mechanical and thermal stability of the retained austenite after ferrite transformation decreases, making it difficult to secure the ferrite + residual austenite complex structure and the appropriate amount of retained austenite, and the ferrite strengthening effect is insufficient to secure the strength. Difficulties also affect delayed fracture resistance, surface corrosion characteristics, impact toughness, permanent deformation during bolt fastening, and ensure uniformity and proper thickness of the surface ferrite decarburized layer in the wire heating furnace for wire decarburization control. This is because it is difficult to perform decarburization, and it is difficult to control surface scale characteristics due to increased hardenability during wire rod cooling. If the silicon exceeds 4.0%, the above-mentioned effect is saturated and adversely affects the hardenability, the composition of the composite tissue steel, the impact toughness, the fatigue characteristics, and the production of bloom or billet for wire rod manufacturing. This is because the quality characteristics in the final product are deteriorated due to the inhomogeneity of the microstructure due to the segregation of silicon at the same time, and it is difficult to control the homogeneous surface decarburization because the thickness of the surface ferrite layer increases during the heat treatment. A more preferable range of the silicon component in the present invention by 2.8-3.3%, bainite structure (ferrite + retained austenite) to the second tank for isothermal heat treatment time and residual austenite fraction and the size, shape, ferrite + retained austenite compound Strengthening and toughening of the structure, delayed fracture resistance (diffuse hydrogen content, precipitation control of grain boundary precipitates), surface decarburization, stress relaxation or permanent strain resistance after bolting, dynamic and static fatigue characteristics This can be improved very effectively.

Manganese (Mn) is an element that forms a solid solution to form a solid to form a solid solution, and is very soluble in high-strength bolt characteristics. Therefore, its content is detrimental to the strength of the base metal, hardenability during heat treatment, stress relaxation, and segregation. It is preferable to set it as 0.1-0.8% in consideration of these. If the content of manganese exceeds 0.8%, local quenchability due to manganese segregation is increased rather than solid solution strengthening effect. Because it is crazy. That is, macro sedimentation and micro segregation are easy to occur depending on the segregation mechanism during steel solidification. Manganese segregation promotes segregation zone due to relatively low diffusion coefficient compared to other elements, which improves the hardenability, which results in the low temperature structure of the core. core martensite). In addition, when less than 0.1% of manganese is added, the formation of segregation zones due to manganese segregation is rare, but the improvement of hardenability, permanent strain resistance and stress relaxation is difficult to expect due to the lack of solid solution strengthening effect.

It is preferable to make content of chromium (Cr) into 0.1 to 0.8%. If the content of chromium is less than 0.1%, 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 quenchability improvement. In addition, if it exceeds 0.8%, it is not preferable because the time required for transformation of ferrite + residual austenite composite structure becomes longer during isothermal heat treatment, and it is difficult to generate a proper surface ferrite layer when charging the wire furnace for wire rod decarburization layer control. This is because it affects decarburization control.

The content of phosphorus (P) and sulfur (S) is preferably 0.01% or less. Phosphorus segregates at grain boundaries and degrades its toughness, limiting its upper limit to 0.01%. Sulfur is a low melting point element that segregates grain boundaries to reduce toughness and form an emulsion, which has a detrimental effect on delayed fracture resistance and stress relaxation characteristics. The upper limit is limited to 0.01%.

The content of nitrogen (N) is preferably made 0.005-0.01%. This is because when the nitrogen content is less than 0.005%, it is difficult to form vanadium and niobium-based nitrides that act as non-diffusion hydrogen trap sites, and when the content exceeds 0.01%, the effect is saturated.

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

Vanadium (V) or niobium (Nb) is an element for improving delayed fracture resistance and stress relaxation resistance, and the addition thereof is preferably 0.01% to 0.5% for one kind or two kinds. If the content is less than 0.01%, 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 precipitation strengthening is expected. This is because it is difficult to improve the stress relaxation resistance is not sufficient, and it is difficult to expect attenuation of austenite grains, which affects the microstructure of the ferrite + residual austenite composite. In addition, when it exceeds 0.5%, the improvement effect on delayed fracture resistance and stress relaxation resistance by V or Nb-based precipitates is saturated, and the amount of coarse alloy carbides which are not dissolved in the base metal during austenite heat treatment increases, such as nonmetallic inclusions. This is because it causes a decrease in fatigue characteristics.

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, and the content thereof is preferably 0.3-2.0%. If the nickel content is less than 0.3%, the surface thickening layer formation is incomplete, so it is difficult to expect the effect of improving the delayed fracture resistance, and the heat treatment time is increased during the spheroidization or graphitization treatment to improve decarburization control, toughness and cold forming. This is because there is no improvement effect of cold forming during bolt forming. If it exceeds 2.0%, the effect is saturated and negatively affects the appropriate amount, size and shape of the amount of retained austenite.

Boron (boron, B) is a grain boundary strengthening element for improving the hardenability and delayed fracture resistance in the present invention, the content is preferably 0.0010 to 0.003%. If the content of boron is less than 0.0010%, the grain boundary strength improvement due to grain boundary strengthening due to grain boundary segregation of boron atoms during heat treatment is insufficient, and the graphitization promoting effect is insufficient during the graphitization treatment to improve cold forming. If the content of boron exceeds 0.003%, the effect is saturated, rather, the precipitation of boron nitride at the grain boundary leads to a decrease in grain boundary strength.

The content of molybdenum (Mo) and tungsten (W) is preferably 0.01-0.5%. If the content is less than 0.01%, the grain boundary strengthening effect of the ferrite and the retained austenite is insufficient, and the hardenability during the heat treatment, the solid solution strengthening of the ferrite, and the Mo and W system precipitation strengthening effects are insufficient. If it exceeds 0.5%, the effect is saturated, and as the hardenability is increased, it is easy to form low-temperature structure (martensite + bainite) during wire rod manufacturing, and the heat treatment time during spheroidization or graphitization treatment to improve cold formability This is because there is a disadvantage.

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

The content of titanium is preferably 0.01-0.2%. If the titanium content is less than 0.01%, the effect of miniaturizing austenite crystal grains is insufficient, and the precipitation distribution of titanium-based carbon and nitrides in the grain boundary effective for delayed fracture resistance is insufficient, so that the improvement effect is difficult to be expected. 0.2% If it exceeds, the additive effect is saturated, and coarse titanium-based carbon and nitride are formed to affect the mechanical properties.

Since the steel formed as described above basically has high-strength characteristics, it can be made of any material such as bolts and springs, and can be given the required physical properties by managing its microstructure through heat treatment suitable for the purpose.

In order to increase the high strength and the resistance to delayed fracture, the steel can be realized by making the microstructure into a ferrite + residual austenite composite. At this time, the residual austenite phase fraction of the composite structure is preferably 15% or more. This means that when the residual austenite phase fraction is less than 15%, tensile strength and yield strength are improved, but elongation, cross-sectional reduction rate, and impact toughness are rapidly increased. This is because there is a problem of deterioration, and martensite is mixed in the structure, which adversely affects the delayed fracture resistance. A more preferable residual austenite phase fraction for showing the effect of the present invention is 20-75%, which is a range in which the delayed fracture resistance can be improved together while achieving high strength.

[Method of Manufacturing Steel Work]

The steel formed as described above is made of any steel processed material, for example, bolts, and then heat-treated (heating, isothermal heat treatment) so that the final fine structure of the bolts is ferrite + residual austenite such that the percentage of residual austenite is 15% or more. )do. Here, the manufacturing method of a steel workpiece is demonstrated using a bolt as an example.

Bloom or steel strip having the component system as described above is hot-rolled into a wire rod and processed into a certain shape to obtain a bolt. In the present invention, the bolt processed through the conventional process as described above is heat-treated according to the present invention.

First, the processed bolt is heated at a temperature above the Ac ₃ transformation point, which is less than the Ac ₃ transformation point, it is difficult to secure the austenite single phase to the abnormal region of ferrite and austenite, so that the ferrite + residual austenite microcomposite to show the effect of the present invention. This is because tissue heterogeneity may result in tissue preparation. This heating temperature is as low as 1150 ℃. If it exceeds 1150 ℃, coarse surface decarburization and austenite grain coarsening may occur during material heating, resulting in quality characteristics (mechanical properties, stress relaxation, surface defects, static Fatigue characteristics, etc.). At this time, the heating time may be heated as long as the time required for the austenitization to be achieved.

It is preferable to heat as mentioned above to make the structure of the bolt into an austenite single phase, and to cool to a temperature of Ms + 50 ° C to Ms + 215 ° C (Ms is the martensite transformation start temperature) at a cooling rate of 20 ° C / sec or more. . This is a problem of longer transformation time to secure ferrite + residual austenite composite structure below Ms + 50 ° C, and high martensite is likely to occur in case of isothermal heat treatment temperature deviation, and affects the appropriate amount of retained austenite, size and shape. This is not preferable because of the fact that the elongation is reduced, and the elongation and impact toughness decrease rapidly. In addition, when Ms + 215 ° C is exceeded, the amount and size, shape, mechanical, and thermal stability of retained austenite are unfavorable for high strength, and due to the rapid decrease in yield ratio (yield strength / tensile strength ratio), proper yield is obtained. This is because there is a problem in securing the strength, and there is a problem in that the stress relaxation is poor when the bolt is fastened, and it is harmful to the fracture resistance by reducing the impact toughness, and also affects the critical delay strength and static fatigue characteristics.

Hereinafter, the present invention will be described in detail by way of examples.

Example 1

Steels having the composition as shown in Table 1 were cast as 50 kg ingot, and then homogenized and heat-treated at 1250 ° C. for 48 hours to obtain hot wires with a thickness of 13 mm. At this time, the finishing temperature was 950 ℃ or more and hot-rolled after hot rolling, the rolling ratio was 80% or more.

The test pieces for evaluating mechanical properties (tensile and impact characteristics) and delayed fracture resistance from the hot rolled materials as described above were taken in the rolling direction of the rolled material. At this time, the heat treatment conditions were tested by heat and isothermal heat treatment conditions shown in Table 2.

division C Si Mn Cr Ni B V Mo Ti W P S N Inventive Steel 1 0.81 2.93 0.33 0.49 - - 0.04 - - - 0.007 0.009 0.006 Inventive Steel 2 0.68 3.54 0.35 0.74 - 0.0010 0.19 - 0.01 - 0.009 0.006 0.012 Invention Steel 3 0.90 3.04 0.29 0.38 0.66 - 0.06 - - 0.03 0.004 0.008 0.008 Inventive Steel 4 0.83 2.09 0.71 0.55 - - 0.12 0.25 0.03 - 0.005 0.004 0.011 Inventive Steel 5 0.82 3.92 0.32 0.32 - 0.0019 0.05 - - 0.18 0.006 0.004 0.008 Inventive Steel 6 1.21 3.11 0.30 0.56 - 0.0013 - 0.04 0.05 0.09 0.007 0.006 0.005 Inventive Steel 7 1.42 2.61 0.79 0.33 1.10 - - 0.10 0.10 - 0.009 0.005 0.005 Comparative Material 1 0.45 3.03 0.29 0.58 - - 0.05 - - - 0.005 0.004 0.008 Comparative Material 2 0.40 3.42 0.31 0.79 - 0.0013 0.2 - 0.01 - 0.006 0.005 0.014 Comparative Material 3 0.60 2.99 0.32 0.33 0.54 - 0.05 - - 0.02 0.007 0.009 0.007 Comparative Material 4 0.45 2.0 0.77 0.51 - - 0.11 0.2 0.03 - 0.006 0.008 0.009 Comparative Material 5 0.44 3.96 0.23 0.27 - 0.0015 0.06 - - 0.2 0.008 0.008 0.008 Comparative Material 6 0.53 3.01 0.35 0.55 - 0.0010 - 0.05 0.05 0.07 0.004 0.009 0.004 Comparative Material7 0.58 2.56 0.80 0.29 1.10 - - 0.13 0.10 - 0.005 0.006 0.005 Comparative Material 8 0.35 0.19 0.67 0.95 0.03 - tr 0.17 - - 0.019 0.015 0.004 Comparative Material 9 0.31 0.20 0.62 0.95 0.04 - tr 0.05 - - 0.017 0.010 0.005 Comparative Material 10 0.34 0.22 0.36 1.26 0.05 - 0.019 0.40 - - 0.011 0.012 0.015 Comparative Material 11 0.20 0.20 0.80 0.72 - 0.0015 - 0.04 - - 0.009 0.004 0.005 Inventive steel 2 contains 0.01% of Nb. Inventive steel 3 contains 0.01% Cu. tr is short for trace.

Inventive steel of Table 1 is heated for 20-30 minutes in the range below 1150 ° C above Ac ₃ transformation point, and 30C / to Ms + 50 ° C to Ms + 250 ° C, which is an isothermal heat treatment temperature range for obtaining ferrite + residual austenite composite structure. Rapid cooling at a cooling rate of sec or more and heat treatment for 60 minutes yielded a phase fraction of residual austenite in the range of 20-75% (Invention of Table 2 (1-11)).

In addition, the comparative steel (1-4) of Table 1 was heated for 40 minutes in the austenite single-phase reverse heating temperature range of 950-1050 ℃, 70 ℃ / up to Ms + 60 ℃ isothermal heat treatment temperature range for bainite transformation The solution was quenched at a cooling rate of sec or more and heat-treated for 40 minutes. In addition, by heating the comparative steel (4-7) at a temperature of Ac ₃ -[(Ac ₃ -Ac ₁ ) / 2], which is the intermediate temperature range of the ideal region, the ferrite phase fraction was prepared in the range of 5-25% and bainite transformation Ms + 80 ℃ for quenching at a cooling rate of more than 70C / sec was prepared by isothermal holding for 40 minutes after oil cooling (Comparative Material (5-8)). In addition, by heating the comparative steel (5-7) at a temperature of Ac ₃ -[(Ac ₃ -Ac ₁ ) / 2], which is the intermediate temperature range of the ideal region, the ferrite phase fraction was prepared in the range of 8-20%, and after cooling with oil 500 It was prepared by tempering at 占 폚 (Comparative Material (12-15)). In addition, the comparative steel (8-11) was heated to austenite single phase in the range 900-950 ° C and tempered at 450 ° C after oil cooling to prepare a conventional tempered martensite structure (Comparative material (12-15)).

The Ms transformation temperatures of these steels were measured using dilatometry and the results are shown in Table 2. In addition, in order to evaluate the tensile characteristics, impact characteristics, and delayed fracture characteristics of the materials manufactured as described above, the tensile test specimens were used in the KS standard (KS B 0801) No. 4 test specimens and the tensile test was cross head speed (cross head). speed) 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 machined from the side of the rolling direction (L-T direction). The residual austenite phase fraction in the microcomposite after heat treatment was measured using X-ray (Cu radiation). 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 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. The delayed fracture tester used a constant loading type delayed fracture testing machine. The delayed fracture test specimen was prepared with a specimen diameter of 6 mmφ, a notch diameter of 4 mmφ, and a notch root radius of 0.1 mm. The test piece atmosphere solution was performed at room temperature (25 ± 5C) at a pH of 2 ± 0.5 with a wolpole buffer solution (HCl + CH3COONa).

The critical delay fracture strength is the tensile strength at which the required time from failure to failure is equal to or greater than 150 hours at the same stress ratio (load stress / notch tensile strength ratio, 0.5), and the notch strength is the maximum load ÷ notch area). The number of specimens for setting the critical delay fracture strength was based on the case where more than 13 undecided specimens were used. Tensile properties and impact toughness of the inventive and comparative agents prepared as described above were measured, and the results are shown in Table 3.

division Target steel grade Heating temperature (℃) Heating time (min) Isothermal heating temperature (℃) Isothermal heating time (min) Phase fraction of retained austenite (%) Transformation temperature (℃) Ac ₃ Ac ₁ Ms Invention 1 Inventive Steel 1 980 30 Ms + 70 60 21 - - 210 Invention 2 980 30 Ms + 100 60 25 - - Invention 3 980 30 Ms + 150 60 49 - - Invention 4 1050 20 Ms + 100 60 26 - - Invention 5 1150 20 Ms + 130 60 40 - - Invention 6 Inventive Steel 2 1050 40 Ms + 100 60 33 - - 224 Invention 7 Invention Steel 3 1050 40 Ms + 150 90 61 - - 193 Invention Material 8 Inventive Steel 4 1050 40 Ms + 150 90 22 - - 177 Invention Material 9 Inventive Steel 5 1000 40 Ms + 150 90 55 - - 214 Invention 10 Inventive Steel 6 1150 40 Ms + 150 90 70 - - 179 Invention 11 Inventive Steel 7 1100 40 Ms + 200 100 75 - - 140 Comparative Material 1 Comparative Steel 1 950 30 Ms + 60 30 - - 290 Comparative Material 2 Comparative Steel 2 1000 40 Ms + 60 40 - - 307 Comparative Material 3 Comparative Steel 3 1000 40 Ms + 60 40 - - 247 Comparative Material 4 Comparative Steel 4 1030 40 Ms + 60 40 - - 278 Comparative Material 5 Comparative Steel 4 x = 2 80 Ms + 80 40 880 782 260 Comparative Material 6 Comparative Steel 5 x = 2 30 Ms + 80 40 961 842 288 Comparative Material7 Comparative Steel 6 x = 2 40 Ms + 80 40 899 817 250 Comparative Material 8 Comparative Steel 7 x = 2 120 Ms + 80 40 857 775 208 Comparative Material 9 Comparative Steel 5 x = 2 30 Tempering treatment at 500 ℃ for 40 minutes 961 842 - Comparative Material 10 Comparative Steel 6 x = 2 40 Tempering treatment at 500 ℃ for 40 minutes 899 817 - Comparative Material 11 Comparative Steel 7 x = 2 120 Tempering treatment at 500 ℃ for 40 minutes 857 775 - Comparative Material 12 Comparative Steel 8 900 30 450 ℃ × 60 minutes tempering - - - Comparative Material 13 Comparative Steel 9 900 30 450 ℃ × 60 minutes tempering - - - Comparative Material14 Comparative Steel 10 950 30 450 ℃ × 60 minutes tempering - - - Comparative Material 15 Comparative Steel 11 900 30 450 ℃ × 60 minutes tempering - - - X is X in the formula of Ac3-[(Ac3-Ac1) / X]

division 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 ) Invention 1 190 155 23 54 79 180 Invention 2 185 148 28 61 91 180 Invention 3 176 142 33 64 77 170 Invention 4 183 145 24 64 88 170 Invention 5 175 135 36 67 95 170 Invention 6 200 163 24 58 98 180 Invention 7 215 172 34 69 107 180 Invention Material 8 198 168 29 68 93 180 Invention Material 9 195 170 30 67 101 180 Invention 10 201 174 32 61 108 180 Invention 11 210 180 35 65 105 180 Comparative Material 1 140 105 15 63 152 140 Comparative Material 2 162 127 15 62 148 150 Comparative Material 3 175 136 16 63 155 150 Comparative Material 4 160 128 15 61 148 150 Comparative Material 5 155 120 33 48 56 150 Comparative Material 6 153 123 30 49 50 150 Comparative Material7 158 124 32 54 56 150 Comparative Material 8 166 129 29 47 50 150 Comparative Material 9 173 137 14 39 37 150 Comparative Material 10 175 147 14 42 42 150 Comparative Material 11 184 164 15 35 37 150 Comparative Material 12 147 135 15 57 30 130 Comparative Material 13 147 129 16 58 20 110 Comparative Material14 148 139 15 57 40 140 Comparative Material 15 110 95 15 60 50 100

As shown in Table 3, the tensile strength of the invention materials (1 to 11) ranges from 175 to 215 kg / mm ^2, and the critical delay fracture strength is 170 to 180 kg / mm ² , but for the comparative materials (1 to 11). Tensile strength is in the range of 140 ~ 184kg / mm ² and critical delay fracture strength is in the range of 140 ~ 150kg / mm ² , and in case of comparative materials (12 ~ 15), tensile strength is in the range of 110 ~ 148kg / mm ² and critical delay fracture strength Is in the range of 100 to 140 kg / mm ² . Thus, it can be seen that the present invention having a ferrite + residual austenite composite structure significantly improved the strength and the critical delay fracture strength compared to the comparative material.

As described above, the present invention proposes an alloy composition system and heat treatment conditions of a composite structure steel composed of ferrite + residual austenite in developing a steel having excellent delayed fracture resistance and high strength, thereby achieving excellent delayed fracture while achieving high strength of the bolt. As the resistance can be secured at the same time, it is possible to provide a micro composite steel for high strength bolts.

Claims (12)

By weight percent carbon 0.65-1.50%, silicon 2.0-4.0%, manganese 0.1-0.8%, chromium 0.1-0.8%, phosphorus and sulfur 0.01% or less, nitrogen 0.005-0.01%, oxygen 0.005% or less Nickel 0.3-2.0%, Boron 0.001-0.003%, Vanadium: 0.01-0.5%, Niobium: 0.01-0.5%, Molybdenum 0.01-0.5%, Titanium 0.01-0.2%, Tungsten 0.01-0.5%, Copper 0.01-0.2% A high strength steel, optionally containing one or two or more of the group consisting of, and excellent in delayed fracture resistance composed of the remaining Fe and other impurities. The high strength steel of claim 1, wherein the carbon is contained in a range of 0.8% to 1.0%. The high strength steel of claim 1, wherein the silicon is contained in a range of 2.8 to 3.3%. The high-strength steel of claim 1, wherein the steel microstructure is a composite structure of ferrite + residual austenite, and the residual stenitite phase fraction is 15% or more. 5. The high strength steel of claim 4, wherein the phase fraction of the retained austenite is 20 to 35%. A bolt having the emphasis of any one of claims 1 to 5. 7. The method of claim 6 wherein the bolts are bolts which features gateum the tensile strength and the critical delayed fracture strength of 170~180kg / mm ² of 175~215kg / mm ^2. By weight, it contains carbon 0.65-1.50%, silicon 2.0-4.0%, manganese 0.1-0.8%, chromium 0.1-0.8%, phosphorus 0.01% or less, sulfur 0.01% or less, nitrogen 0.005-0.01%, oxygen 0.005% or less Here, nickel 0.3-2.0%, boron 0.001-0.003%, vanadium: 0.01-0.5%, niobium: 0.01-0.5%, molybdenum 0.01-0.5%, titanium 0.01-0.2%, tungsten 0.01-0.5%, copper 0.01- A steel workpiece containing one or two or more selected from the group consisting of 0.2% and consisting of the remaining Fe and other impurities is heated at a temperature of Ac ₃ transformation point or more to form austenite single phase and at a cooling rate of 20 ° C./sec or more. Excellent delayed fracture resistance including rapid quenching to the temperature of Ms + 50 ℃ ~ Ms + 215 ℃ and isothermal heat treatment for more than 60 minutes and oil or air cooling to have a composite structure of residual austenite and ferrite with an ordinary percentage of 15% or more Fabrication of high strength steel products with ferrite + residual austenite composite . The method according to claim 8, wherein the carbon is contained in a range of 0.8% to 1.0%. 10. The method of claim 8, wherein the silicon is contained in a range of 2.8 to 3.3%. The method of claim 8, wherein the phase ratio of the retained austenite is 20 to 35%. The method according to any one of claims 8 to 11, wherein the steel workpiece is a bolt, and has a ferrite + residual austenite composite structure having excellent delayed fracture resistance.