KR100946130B1 - Method of manufacturing high carbon Working product having superior delayed fracture resistance - Google Patents

Abstract

The present invention relates to a method of manufacturing a wire rod into a high carbon steel workpiece such as bolts and springs. This production method is 0.65-1.50% carbon, 2.0-4.0% silicon, 0.1-0.8% manganese, 0.01% or less phosphorus, 0.01% or less sulfur, 0.004-0.013% nitrogen, 0.005% or less oxygen (0% One or two or more selected from the group consisting of boron 0.001-0.004%, titanium 0.005-0.03%, nickel 0.3-2.0%, vanadium: 0.01-0.5%, niobium: 0.01-0.5% , The remaining Fe and other impurities, the titanium (Ti), nitrogen (N), boron (B) is the following relationship, 0.5≤Ti / N≤2.0, 2≤N / B≤8, 1.0≤ (Ti Graphitizing and heat-cooling the wire rod satisfying + 5B) /N≦3.5 at a temperature of Ac ₁ − (60 ± 30 ° C.),

Cold forming the graphitized heat treated wire rod into a steel workpiece,

The steel workpiece is heated to a temperature above the Ac ₃ transformation point to obtain austenite single phase, quenched to a temperature range of Ms + 50 ° C to Ms + 215 ° C at a cooling rate of 20 ° C / sec or more, and then isothermally treated and cooled for 60 minutes or more. And a heat treatment step of having a composite structure of residual austenite and the remaining ferrite having an ordinary percentage of 15% or more.

High carbon, bolt, spring, delayed fracture, graphitized

Description

Method of manufacturing high carbon Working product having superior delayed fracture resistance}

The present invention relates to a method for manufacturing a wire rod with a high carbon steel workpiece such as bolts and springs, and more particularly, by using a composite structure characteristic of ferrite and residual austenite while shortening the graphitization heat treatment time to secure cold formability. Therefore, the present invention relates to a method for manufacturing a steel workpiece excellent in critical delay strength and other mechanical properties.

Bolts require high strength for fastening members for efficient construction of steel structures and for weight reduction, multifunction and high performance of automobile parts. However, there is a problem in that the delayed fracture resistance deteriorates with the increase of the high strength, so that the bolt cannot be used with a tensile strength of 130 kg / mm ² or more, and its use and range are limited.

On the other hand, the cold forming process is used to process the wire into a variety of steel, such as bolts, in the case of high-strength material, it is necessary to secure the tensile strength of less than 60kg / mm ² through the material softening heat treatment before cold forming. This is to suppress the increase of the die wear rate during cold forming to the maximum. Domestic steel structure fastening bolts are currently obtained by cold forming wire rods having a tensile strength of 60 kg / mm ² or less. Therefore, in order to use a high-strength bolt material, it is necessary to secure not only excellent delayed fracture resistance but also cold forming required during bolt manufacturing.

Softening heat treatment for securing cold formability is mostly applied to the spheroidization heat treatment method, characterized in that the microstructure is composed of ferrite + cementite. However, there is a problem in that the strength of the spheroidized material increases as the amount of alloy element added increases, and thus there is a limit in microstructure configuration to overcome this problem.

Conventional bolting materials are mostly microstructured quasi single phase structure of tempered martensite, and Fe-based precipitates are distributed at grain boundaries and precipitates are also distributed on the base material of lath martensite. Can be seen as a general feature. However, it is known that the precipitate deposited at the grain boundaries acts as a trapped site of hydrogen and degrades the strength of the grain boundary, so that the delayed fracture resistance is lowered and the strength of the material cannot be achieved. Therefore, it is accepted that the microstructure of tempered martensite has a limit in achieving high strength.

In order to achieve high strength without deteriorating the bolt's delayed fracture resistance, it is most important to minimize the distribution of Fe-based precipitates that are distributed at grain boundaries after heat treatment.

In view of this aspect, the present inventors have proposed that the steel having a fine structure of bainite or ferrite + bainite is used to improve the delay fracture resistance to 150kg / mm2, according to Korean Patent Application Nos. 98-50898 and 98. Proposed in -50899.

Korean Patent Application No. 98-50898 discloses, in weight percent, carbon 0.40-0.60%, 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% , Containing 0.005% or less of oxygen, including 0.3-2.0% nickel, 0.001-0.003% boron, 0.01-0.5% vanadium, 0.01-0.5% niobium, 0.01-0.5% molybdenum, 0.01-0.2% titanium 0.01-0.5%, copper 0.01-0.2%, cobalt 0.01-0.5% selected from the group consisting of one or two or more of the remaining Fe and other impurities, the steel work is heated at a temperature of Ac ₃ transformation point for more than 20 minutes It is made into austenite single phase, and it is quenched to Ms + 30 ℃ ~ Ms + 60 ℃ at the cooling rate of 70 ℃ / sec or higher, and then transformed at constant temperature for more than 20 minutes so that the microstructure has bainite. ^Two levels of forced defense have been proposed.

The Republic of Korea Patent Application No. 98-50899 discloses the Republic of Korea Patent Application No. 98-50898 above the Ac ₃ to the same steel component with the call station Temperature Range _{- (Ac 3 -Ac 1) Ac} 3 in /1.3 - (Ac ₃ -Ac ₁ ) heating at least for 20 minutes in the range of 5.5 to form a composite structure (ferrite and austenite) having a ferrite phase fraction of 5-25% or an austenite phase fraction of 75-95%, followed by 70 ° C / sec. After quenching to Ms + 50 ℃ ~ Ms + 110 ℃ with the above cooling rate, it is transformed to constant temperature for 20 minutes or more so that the microstructure is a composite structure of ferrite and bainite and the ferrite phase fraction has 5-25%. A method for producing steel with a strength of 150 kg / mm ² has been proposed.

Prior arts of Korean Patent Application Nos. 98-50898 and 98-50899 have improved the critical delay fracture strength to the level of 150 kg / mm 2, but there is no mention of bolt cold forming. The 98-50898 has an elongation of 15%, the 98-50899 has a cross-sectional reduction of less than 60% and the impact toughness of 110 J / cm ² , which deteriorates certain mechanical properties. It has a technical limitation that it cannot improve more than 150kg / mm2.

The present invention has been completed in a series of research process to overcome the technical limitations of the prior art, the purpose is to reduce the graphitization heat treatment time to secure cold formability and the critical delay fracture strength of 150kg / mm ² or more The present invention provides a method for manufacturing a high carbon steel product having a composite structure of residual austenite and ferrite, which is further improved while improving other mechanical properties.

Steel workpiece manufacturing method 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%, phosphorus 0.01% or less, sulfur 0.01% or less, nitrogen 0.004-0.013 %, Oxygen 0.005% or less (excluding 0%), boron 0.001-0.004%, titanium 0.005-0.03%, including nickel 0.3-2.0%, vanadium: 0.01-0.5%, niobium: 0.01-0.5% It is composed of one or two or more selected from the group consisting of, the remaining Fe and other impurities, the titanium (Ti), nitrogen (N), boron (B) is the following relationship, 0.5≤Ti / N≤2.0, 2≤ Graphitizing and heat-cooling the wire rod satisfying N / B ≦ 8, 1.0 ≦ (Ti + 5B) /N≦3.5 at a temperature of Ac ₁ − (60 ± 30 ° C.),

Cold forming the graphitized heat treated wire rod into a steel workpiece,

The steel workpiece is heated to a temperature above the Ac ₃ transformation point to obtain austenite single phase, quenched to a temperature range of Ms + 50 ° C to Ms + 215 ° C at a cooling rate of 20 ° C / sec or more, and then isothermally treated and cooled for 60 minutes or more. And a heat treatment step of having a composite structure of residual austenite and the remaining ferrite having an ordinary percentage of 15% or more.

Hereinafter, the present invention will be described in detail.                     

In the present invention, the term 'steel workpiece' refers to a workpiece obtained by processing a wire rod in a predetermined form, and includes all steel workpieces applied to applications in which the physical properties of the steel of the present invention can be used. For example, there are bolts, nuts, springs and the like that can be produced by processing the wire rod.

The inventors of the present invention can secure excellent cold formability regardless of the addition of alloying elements in the material tensile strength or surface hardness compared to the existing spheroidizing heat treatment method in the case of using the graphitized structure in cold forming high carbon steel workpiece (bolt). The present invention is based on the result that the graphitization heat treatment time can be minimized when the composition ratio between alloy elements is 0.5≤Ti / N≤2.0, 2≤N / B≤8, 1.0≤ (Ti + 5B) /N≤3.5. Will be proposed. In the present invention, the wire rod is graphitized and heat-cooled at a temperature of Ac _1- (60 ± 30 ° C) and air cooled to have a graphite grain size of 50 µm or less and a graphite grain phase fraction of 0.1% or more, thereby reducing die wear rate during bolt cold forming. Solve the problem easily.

In addition, the present invention fundamentally suppress the precipitation of grain boundary precipitates (Fe-based) harmful to the delayed fracture resistance at the austenite grain boundary, while developing a steel having excellent mechanical properties while maintaining a critical delay fracture strength of 150 kg / mm ² or more. In order to achieve this, multi-faceted studies on microstructural control have led to the following conclusions. In high carbon steels, the wires with a silicon content in the range of 2.0% to 4.0% are cold formed into steel products and isothermally treated to have a fine composite structure (ferrite + residual austenite).

(1) the ferrite and the retained austenite have a lamellar distribution similar to that of pearlite, and the spacing between these phases can be considerably fined in the range of about 0.1 to 0.3 μm to achieve further high 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) Bolted steel with more than 150kg / mm ² critical delay fracture strength can be manufactured which can achieve high reinforcement with excellent delayed fracture resistance by retained austenite in composite structure. From this point of view, it will be described by dividing the emphasis of the present invention and the manufacturing method of the steel workpiece.

[Lecture composition]

The content of carbon (C) is preferably 0.65-1.50%.

If the carbon content is less than 0.65%, it is difficult to obtain the appropriate amount, shape and size of the retained austenite in the ferritic + residual austenite composite structure after heat treatment for the production of the ferritic + residual austenite composite steel. In addition, it is difficult to secure mechanical and thermal stability, and it is difficult to secure sufficient tensile strength and yield strength as high-strength bolt steel. If the content of carbon exceeds 1.50%, the appropriate cross-sectional reduction rate, elongation and impact toughness decrease after heat treatment.

The content of silicon (Si) is preferably limited to 2.0-4.0%.

If the silicon is less than 2.0%, the graphitization heat treatment time for improving the cold forming property is long, and the mechanical and thermal stability of the retained austenite is degraded after the ferrite transformation. It is difficult to secure the amount of nitrate, and it is difficult to secure the strength due to the insufficient effect of strengthening ferrite. Also, it is delayed fracture resistance, surface corrosion characteristic, impact toughness, bainite transformation structure, permanent deformation during bolting, wire surface decarburization control. Affects the back If the content of silicon is more than 4.0%, the above-mentioned effects are not preferable because they saturate and affect hardenability, microstructure, impact toughness, fatigue properties, and the like.

As for manganese (Mn), 0.1 to 0.8% is preferable.

Manganese is an element that forms a solid solution to form a solid solution and strengthens solid solution.It is very useful for high-strength bolt characteristics, so it is 0.1% or more in consideration of the strength of the base material, the hardenability during heat treatment, the stress relaxation, and the harmful effects of segregation. It is preferable to add. When the content of manganese exceeds 0.8%, local quenchability due to manganese segregation is increased rather than solid solution strengthening effect, and the anisotropy of tissue is intensified by the formation of segregation zone. Crazy

The content of phosphorus (P) and sulfur (S) is preferably 0.01% or less, respectively.                     

Phosphorus segregates at grain boundaries and lowers its toughness, limiting its upper limit to 0.01%. Sulfur is a low melting point element that segregates grains to reduce toughness and form 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%.

The content of nitrogen (N) is preferably set to 0.004-0.013%.

Nitrogen is less than 0.004% softening heat treatment (graphitization) effect to improve the bolt cold formability, it is difficult to form vanadium and niobium-based nitride acting as a non-diffusing water trap site, and when it exceeds 0.013% graphitization It is not preferable because the time is long.

The content of oxygen (O) is preferably 0.005% or less (excluding 0%).

When the oxygen content exceeds 0.005%, coarse oxide-based nonmetallic inclusions are easily formed and fatigue life is reduced.

As for boron (boron, B), 0.001 to 0.004% is preferable.

Boron is a grain boundary hardening element for improving hardenability and delayed fracture resistance. If the boron content is less than 0.001%, the graphitization promoting effect and the grain boundary segregation of boron atoms are insufficient during heat treatment, so that the grain boundary strength improvement is not large, and the graphitization promoting effect is insufficient when the graphitization treatment is performed to improve cold formability. In addition, when the content of boron exceeds 0.004%, the effect is saturated, rather, precipitation of boron nitride in the grain boundary causes a decrease in grain boundary strength.

The content of titanium is preferably made 0.005-0.03%.

If the content of titanium is less than 0.005%, the effect of promoting graphitization and miniaturization of austenite grains is insufficient, and the precipitation distribution of titanium-based carbon and nitride in the grain boundary effective for delayed fracture resistance is insufficient. In addition, when it exceeds 0.03%, the effect is saturated, and coarse titanium-based carbon and nitride are formed to affect the mechanical properties.

In the present invention, the tensile strength required for cold forming before bolt forming (typically 60kg / mm ² or less) is to be achieved by graphitizing the material, and the composition ratio between alloy elements is 0.5 to significantly shorten the time required for the graphitizing of the material. It is limited to ≤ Ti / N ≤ 2.0, 2.0 ≤ N / B ≤ 8.0, 1.0 ≤ (Ti + 5B) / N ≤ 3.0, for the following reasons.

As for Ti / N ratio, 0.5 <= Ti / N <= 2.0 is preferable.

If the Ti / N ratio is less than 0.5, the graphene grain formation rate decreases due to the decrease of nucleation sites of the graphite grains, and the graphite grains tend to coarsen when the Ti / N ratio exceeds 2.0.

The N / B ratio is preferably 2.0 ≦ N / B ≦ 8.0.

If the N / B ratio is less than 2.0, the number of BN precipitates necessary for graphite nucleation is insufficient, and the graphitization rate is lowered. If the N / B ratio is greater than 8.0, the number of BN precipitates required for graphite nucleation is saturated, and the amount of nitrogen employed in the base metal is saturated. The graphitization rate is lowered rather than increasing.

As for (Ti + 5B) / N ratio, 1.0 <(Ti + 5B) / N <= 3.5 is preferable.

If the (Ti + 5B) / N ratio is less than 1.0, the number of TiN and BN precipitates that contribute to the nucleation of the graphite grains becomes insufficient, and if the (Ti + 5B) / N ratio exceeds 3.5, the TiN and BN precipitates required for the graphite grain nucleation The number is saturated and the amount of nitrogen dissolved in the base material increases, which adversely affects the graphitization rate.

One or two or more of nickel, vanadium and niobium are added to the above components.

Nickel (Ni) is preferably 0.3 to 2.0%.

Nickel is an element that forms a nickel thickening layer on the surface during heat treatment to suppress permeation of external hydrogen to improve delayed fracture resistance. 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 the decarburization control, toughness and cold forming. There is no effect of improving cold forming during bolt forming. When the nickel content exceeds 2.0%, the effect is saturated and affects the appropriate amount, size and shape of the amount of residual austenite.                     

The vanadium (V) or niobium (Nb) is preferably 0.01 to 0.5%, respectively.

Vanadium or niobium is an element that improves delayed fracture resistance and stress relaxation. 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. It is difficult to improve the stress relaxation resistance is not sufficient, and it is difficult to expect austenite grain refinement, which affects the structure refinement when constructing the ferrite + residual austenite composite. In addition, if it exceeds 0.5%, the graphitization heat treatment time is long, and the improvement effect on delayed fracture resistance and stress relaxation resistance by V or Nb-based precipitates is saturated and is not dissolved in the base material during austenite heat treatment. Coarse alloy carbide increases, which acts like non-metallic inclusions, resulting in lower fatigue properties.

[Method for Manufacturing Steel Work]

In the present invention, the wire rod is cold-formed to make a steel workpiece (such as bolts), and the steel workpiece is heat-treated to obtain a composite structure.

[Wire Rod Manufacturing Process]

Generally, the wire is obtained by billet rolling the bloom or by directly rolling and cooling the billet obtained by casting directly, and then the wire is fresh. The spheroidization heat treatment is performed before and after drawing.

In the present invention, the wire rod having the above-described steel component system is subjected to the graphitization heat treatment during the cooling process or before and after the heat treatment. The graphitization heat treatment is performed at Ac _1- (60 ± 30 ° C). This temperature range is the fastest graphitization rate and shows the fastest graphitization behavior at Ac ₁ -60 ℃. Therefore, when the graphitization heat treatment temperature is lower than Ac ₁ -90 ℃, there is a problem in securing the productivity with a temperature range that the graphitization rate is very slow, and at a temperature above Ac ₁ -30 ℃, the graphitization heat treatment time is long and the austenite phase is longer. Precipitation is likely to be reusable. When the graphitization heat treatment is performed according to the present invention, the wire rod has a graphite grain size of 50 µm or less, and its phase fraction becomes 0.1% or more. When the size of the graphite grains is larger than 50 μm, there is a problem of causing surface defects rather than the effect of improving the cold forming property, and when the graphite grain percentage is less than 0.1%, there is no softening effect of the tissue for improving the cold forming property.

When the graphitization heat treatment is performed in the process of rolling and cooling the wire rod according to the present invention, since the cold forming of the wire rod is secured, the spheroidization heat treatment for drawing or cold forming may be omitted.

[Cold forming process]

The wire rod provided in accordance with the present invention is cold formed by bolts, springs, etc. in a conventional manner to obtain a steel workpiece.

[Compound structure heat treatment process]                     

Cold-formed steel products are heat-treated to secure the desired microstructure.

In the heat treatment, first, the steel workpiece is heated above the Ac ₃ transformation point to secure an austenite single phase. Under Ac ₃ transformation point, it is difficult to secure a single phase of austenite as an abnormal region of ferrite and austenite, which may result in tissue non-uniformity when manufacturing a ferrite + residual austenite composite tissue for showing the effect of the present invention. This heating temperature is below 1150 ℃. If it exceeds 1150 ℃, it causes surface decarburization and coarsening of austenite grains when heating the material, so that the quality characteristics (mechanical properties, stress relaxation, surface scratches, static fatigue characteristics) in the final product Etc.). The heat treatment is sufficiently heated to obtain the desired austenite single phase, and heating for about 20 minutes or more can complete the desired transformation.

After heating as described above, it is quenched at a cooling rate of 20 ° C./sec or more, and then isothermally heat-treated and cooled in a temperature range of Ms + 50 ° C. to Ms + 215 ° C. to prepare ferrite + residual austenite composite structure. Cooling is oil or air cooled. If the isothermal heat treatment temperature is below Ms + 50 ℃, the transformation time to secure the ferrite + residual austenite composite structure is long, and martensite is highly likely to occur when isothermal heat treatment temperature deviation occurs, and the appropriate residual austenite amount, size and shape It is not preferable because of the influence, and the elongation and impact toughness decrease rapidly. In addition, if it exceeds Ms + 200 ° C, the amount and size, shape, mechanical and thermal stability of retained austenite are unfavorable for high strength, and it is appropriate for rapid reduction of yield ratio (yield strength / tensile strength ratio). There is a problem in securing the yield strength, and there is a problem in that the stress relaxation property is inferior when bolting, and it is harmful to the fracture resistance by reducing the impact toughness, and also affects the critical delay fracture strength and the static fatigue characteristics.

The isothermal heat treatment according to the present invention results in a microstructure of a composite structure of residual austenite + ferrite. 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. It is degraded, and martensite is mixed in the structure, which has a detrimental effect on 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.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1

Steels having the composition as shown in Table 1 were cast into 50 kg ingots, and then homogenized and heat-treated at 1250 ° C. for 48 hours to hot roll 13 mm in thickness to prepare wires. At this time, the finishing temperature was 950 ℃ or more and air-cooled after hot rolling. The rolling ratio was 80% or more.                     

division Chemical composition (% by weight) C Si Mn Cr V Ni Ti B P S N Nb O Invention steel One 0.79 3.08 0.52 - 0.05 - 0.008 0.0020 0.009 0.009 0.0055 0.03 0.0013 2 0.72 3.48 0.53 - 0.06 - 0.01 0.0012 0.007 0.008 0.0072 0.0014 3 0.88 3.10 0.57 - 0.07 0.72 0.013 0.0011 0.006 0.009 0.0064 0.0015 4 0.86 2.21 0.8 - 0.01 - 0.01 0.0040 0.006 0.009 0.0093 0.0016 5 0.85 3.92 0.52 - 0.04 - 0.005 0.0019 0.007 0.006 0.0044 0.0017 6 1.21 3.25 0.63 - - - 0.013 0.0023 0.007 0.007 0.0120 0.0017 7 1.38 2.55 0.8 - - 1.20 0.02 0.0034 0.009 0.008 0.0118 0.0018 Comparative steel One 0.81 2.93 0.33 0.49 0.04 - - - 0.007 0.009 0.006 0.0012 2 0.68 3.54 0.35 0.74 0.19 - 0.01 0.001 0.009 0.006 0.012 0.0015 3 0.90 3.04 0.29 0.38 0.06 0.66 - - 0.004 0.008 0.008 0.0017 4 0.83 2.09 0.71 0.55 0.12 - 0.03 - 0.005 0.004 0.011 0.0013 5 0.82 3.92 0.32 0.32 0.05 - - 0.0019 0.006 0.004 0.008 0.0015 6 1.21 3.11 0.30 0.56 - - 0.05 0.0013 0.007 0.006 0.005 0.0016 7 1.42 2.61 0.79 0.33 - 1.10 0.10 - 0.009 0.005 0.005 0.0018

division Ti / N N / B (Ti + 5 × B) / N Inventive Steel 1 1.5 2.8 3.3 Inventive Steel 2 1.4 6.0 2.2 Invention Steel 3 2.0 5.8 2.9 Inventive Steel 4 1.1 2.3 3.2 Inventive Steel 5 1.1 2.3 3.3 Inventive Steel 6 1.1 5.2 2.0 Inventive Steel 7 1.7 3.5 3.1 Comparative Steel 1 - - - Comparative Steel 2 0.83 12.00 0.43 Comparative Steel 3 - - - Comparative Steel 4 2.73 Comparative Steel 5 - 4.21 - Comparative Steel 6 10.00 3.85 1.35 Comparative Steel 7 20.00 - -

Specimens for evaluating mechanical properties (tensile and impact properties) from hot rolled materials were taken from the rolling direction of the rolled material. The graphitized heat treatment of the inventive steel was maintained at 750 ° C. under the conditions of Table 3, and the spheroidization heat treatment of the comparative steels was held at 830 ° C. for 5 hours, followed by slow cooling at 10 ° C./hr to 650 ° C., followed by air cooling. The mechanical properties of the inventive and comparative materials prepared as described above were measured, and the results are shown in Table 3.                     

division Graphitization Time (min) Graphite Grain Percentage (%) Mechanical properties after graphitization or spheroidization heat treatment Tensile Strength (kg / ㎠) Yield strength (kg / ㎠) Elongation (%) Cross section reduction rate (%) Impact Toughness (J / ㎠) Inventive Steel 1 7 2.1 57 40 31 54 70 Inventive Steel 2 9 2.0 54 38 36 59 83 Invention Steel 3 10 2.2 59 41 30 49 79 Inventive Steel 4 8 2.3 54 39 36 63 83 Inventive Steel 5 14 2.4 58 39 32 60 70 Inventive Steel 6 10 2.1 58 36 35 62 78 Inventive Steel 7 8 2.0 59 41 33 65 84 Comparative Steel 1 - - 78 57 32 53 62 Comparative Steel 2 - - 77 58 33 56 64 Comparative Steel 3 - - 81 63 29 58 58 Comparative Steel 4 - - 83 65 27 63 54 Comparative Steel 5 - - 79 62 30 59 60 Comparative Steel 6 - - 84 63 25 53 57 Comparative Steel 7 - - 88 65 26 63 54

The wire rod having the graphitized structure shown in Table 3 was heated in the range of Ac ₃ transformation point, which is the austenite single-phase induction heating range, above 1150 ° C, and 30 ° C / to the isothermal heat treatment temperature range to obtain a composite structure of ferrite and residual austenite It was quenched at a cooling rate of more than sec and isothermally treated. At this time, the heat treatment conditions are shown in Table 4.

The mechanical properties of the materials prepared as described above were evaluated.

(1) Tensile test and impact test

Tensile test pieces were tested using a KS standard (KS B 0801) No. 4 test piece at a cross head speed of 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).

(2) tissue percentage                     

The total residual austenite phase fraction in the microcomposite after heat treatment was measured using X-ray (Cu radiation). Respective phase fractions for the amount of retained austenite of the strip like type and the brachy type within the graphite grain distribution and the total residual austenite fraction were measured using the point counting method. In addition, the austenite grain size was measured by the KS standard (KS D 0205).

(3) threshold delay strength

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 notched 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 undecided specimens were used.                     

division Target steel grade Heating temperature (℃) Heating time (min) Isothermal heating temperature (℃) Isothermal Heat Treatment Time (min) Percent fraction of retained austenite (%) Transformation temperature (℃) Ms Invention 1 Inventive Steel 1 980 30 Ms + 70 60 22 216 Invention 2 980 30 Ms + 100 60 23 Invention 3 980 30 Ms + 150 60 53 Invention 4 1050 20 Ms + 100 60 24 Invention 5 1150 20 Ms + 130 60 41 Invention 6 Inventive Steel 2 1050 40 Ms + 100 60 32 227 Invention Material7 Invention Steel 3 1050 40 Ms + 150 90 59 196 Invention Material 8 Inventive Steel 4 1050 40 Ms + 150 90 19 181 Invention Material 9 Inventive Steel 5 1000 40 Ms + 150 90 53 218 Invention 10 Inventive Steel 6 1150 40 Ms + 150 90 72 182 Invention Material 11 Inventive Steel 7 1100 40 Ms + 200 100 73 143 Comparative Material 1 Comparative Steel 1 980 30 Ms + 70 60 21 210 Comparative Material 2 980 30 Ms + 100 60 25 Comparative Material 3 980 30 Ms + 150 60 49 Comparative Material 4 1050 20 Ms + 100 60 26 Comparative Material 5 1150 20 Ms + 130 60 40 Comparative Material 6 Comparative Steel 2 1050 40 Ms + 100 60 33 224 Comparative Material7 Comparative Steel 3 1050 40 Ms + 150 90 61 193 Comparative Material 8 Comparative Steel 4 1050 40 Ms + 150 90 22 177 Comparative Material 9 Comparative Steel 5 1000 40 Ms + 150 90 55 214 Comparative Material 10 Comparative Steel 6 1150 40 Ms + 150 90 70 179 Comparative Material 11 Comparative Steel 7 1100 40 Ms + 200 100 75 140

Mechanical properties after isothermal heat treatment division Tensile Strength (kg / ㎠) Yield strength (kg / ㎠) Elongation (%) Cross section reduction rate (%) Impact Toughness (J / ㎠) Critical Delay Fracture Strength (kg / ㎠) Invention 1 189 157 23 52 75 175 Invention 2 184 148 27 60 90 180 Invention 3 179 143 33 62 74 175 Invention 4 183 144 28 66 85 175 Invention 5 178 138 34 62 98 170 Invention 6 205 165 25 56 93 175 Invention Material7 210 178 30 66 110 175 Invention Material 8 199 168 29 67 97 185 Invention Material 9 195 166 32 69 100 180 Invention 10 208 175 31 62 103 175 Invention Material 11 215 176 33 63 102 185 Comparative Material 1 190 155 23 54 79 180 Comparative material 2 185 148 28 61 91 180 Comparative material 3 176 142 33 64 77 170 Comparative material 4 183 145 24 64 88 170 Comparative material 5 175 135 36 67 95 170 Comparative Material 6 200 163 24 58 98 180 Comparative material 7 215 172 34 69 107 180 Comparative Material 8 198 168 29 68 98 180 Comparative material 9 195 170 30 67 101 180 Comparative Material 10 201 174 32 61 108 180 Comparative Material 11 210 180 35 65 105 180

As shown in Tables 4 and 5, it can be seen that the mechanical properties of the inventive materials are equal to or greater than those of the comparative materials.

As described above, the present invention can significantly improve the cold formability by a short graphitization heat treatment, and the critical delay fracture strength, elongation and cross-sectional reduction rate, impact toughness, etc. markedly improved through the control of the composite structure. There is a useful effect that can provide a steel workpiece.

Claims (3)

By weight% carbon 0.65-1.50%, silicon 2.0-4.0%, manganese 0.1-0.8%, phosphorus 0.01% or less, sulfur 0.01% or less, nitrogen 0.004-0.013%, oxygen 0.005% or less (excluding 0%), boron 0.001 -0.004%, titanium contains 0.005-0.03%, including one or more selected from the group consisting of 0.3-2.0% nickel, 0.01-0.5% vanadium, 0.01-0.5% niobium, the rest Fe and other Titanium (Ti), nitrogen (N), and boron (B) are composed of impurity, and the following relationship: 0.5≤Ti / N≤2.0, 2≤N / B≤8, 1.0≤ (Ti + 5B) / N Graphitizing and heat-cooling the wire rod satisfying ≤3.5 at a temperature of Ac _1- (60 ± 30 ℃), Cold forming the graphitized heat treated wire rod into a steel workpiece, The steel workpiece is heated to a temperature above the Ac ₃ transformation point to obtain austenite single phase, quenched to a temperature range of Ms + 50 ° C to Ms + 215 ° C at a cooling rate of 20 ° C / sec or more, and then isothermally treated and cooled for 60 minutes or more. A method for producing a high carbon steel workpiece having excellent delayed fracture resistance, comprising a heat treatment step of having a composite structure of residual austenite and residual ferrite having a phase ratio of 15% or more. The method of claim 1, wherein the graphitization heat treatment is within 15 minutes. The method of claim 1, wherein the phase ratio of the retained austenite is 20 to 75%.