KR20010060758A - Method for manufacturing medium carbon wire rod containing high silicon without low tempreature structure - Google Patents

Abstract

PURPOSE: A manufacturing method for a medium-carbon steel strip added with high-silicon having no low temperature organization is provided to cool the steel strips in a general cooling equipment under control without employing any EDC(Easy Drawing conveyor) equipments or control rolling equipments, for restraining the producing of a low temperature organization, thereby guaranteeing fine ferrite+pearlite organization with a uniform quality. CONSTITUTION: A manufacturing method for a medium-carbon steel strip added with high-silicon having no low temperature organization includes the steps of heating a billet consisting of carbon 0.40-0.60wt%, silicon 2.0-4.0wt%, manganese 0.1-0.8wt%, chrome 0.1-0.8wt%, phosphorus 0.01wt% or less, sulfur 0.01wt% or less, nitrogen 0.005-0.01wt%, oxygen 0.005wt% or less, one or two selected from a group consisting of nickel 0.3-2.0wt%, boron 0.001-0.003wt%, vanadium 0.01-0.5wt%, niobium 0.01-0.5wt%, molybdenum 0.01-0.5wt%, titanium 0.01-0.2wt%, tungsten 0.01-0.5wt%, and copper 0.01-0.2wt%, Fe and other impurities at a temperature of 950-1150°C for carrying out hot rolling of strips, rapidly cooling the rolled strips by water-spraying to a temperature of 760-790°C to wind, air-cooling the strips at a cooling speed of 1.4±0.4°C/sec to a temperature of 670±30°C, slowly cooling the strips at a cooling speed of 0.6±0.3°C/sec to a temperature of 595±65°C, and air-cooling the strips.

Description

Method for manufacturing medium carbon wire rod containing high silicon without low tempreature structure}

The present invention relates to a method for manufacturing wire rods processed by bolts and the like, and more particularly, to a method for manufacturing high-silicon-added medium-carbon steel wire rods without low-temperature structure occurring during cooling after hot wire rolling due to high amount of silicon addition. It is about.

Wire rod is processed to a certain shape and used for various mechanical parts, for example, bolts, nuts, springs and the like. In order to reduce the weight and performance of such mechanical parts, demand for increasing the strength of wire rods continues to increase. High-strength materials can cause delayed fracture, in which cracks are propagated by hydrogen under constant load.

For example, the bolt is a problem that the delayed fracture resistance deteriorates, it is currently impossible to use more than 130kg / mm ² grade tensile strength is the situation that the use and range is limited. The followings are the benefits of developing high strength steels 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. 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 delayed fracture resistance of the material, the spreading degree is expected to be considerably large considering the advantages in use and the effect on the industry.

The delayed fracture resistance of high-strength materials is known to be lowered because the precipitates distributed at the grain boundaries act as trapped sites of hydrogen and degrade the strength of the grain boundaries. It is most important to suppress the distribution of Fe-based precipitates to be distributed to the maximum. Accordingly, the present inventors have developed a high-strength, high-silicon-added medium-carbon steel excellent in delayed fracture resistance because there is no possibility of precipitation of Fe-based precipitates at grain boundaries.

However, the high-silicon-added medium carbon steel has Ac ₁ and Acm transformation points depending on the Si component range compared to the low-silicon-added steel, but it is roughly 50 ~ 200 ℃ or more, so it is easy to cool the wire structure (bainite or martensite). There is a characteristic that is generated. If the low temperature structure exists in the wire rod, stress is concentrated around the low temperature structure during wire drawing for wire diameter adjustment of the bolt wire rod, which is the main cause of disconnection, which leads to the deterioration of the freshness. There is a point. In addition, there is a problem that it is difficult to secure the proper surface hardness required for cold forming because the spheroidization heat treatment for cold forming can lead to heterogeneous homogenization of the globular structure. Therefore, in order to manufacture high-silicon-added heavy carbon steel as a wire rod, wire rods must be manufactured without the occurrence of low-temperature structure in the wire rod rolling process to ensure fresh workability.

Typically, the wire is cooled while being transferred from the Stelmore facility after high-speed hot rolling, in this case, after the rapid cooling to 880 ℃ by water spraying and winding in the form of a coil, the average cooling finish temperature 0.8 ℃ / sec average cooling finish temperature It cools slowly to near 720 degreeC and air-cools. In the conventional hot wire rolling process, the cooling control technology (cooling temperature and cooling rate) is not only very difficult to secure ferrite + perlite structure without low temperature structure, but also completely suppresses low temperature structure (bainite + martensite). It is almost impossible to do. This is due to the ability of the Stelmoir facility. In the Steamoir facility, when the wire rod is transformed from austenite to ferrite + perlite, the low-temperature structure of the unmodified residual austenite is cooled when the wire rod is air-cooled (bainite + martensite). This is because it is transformed into). Of course, the use of an EDC (easy drawing conveyor) facility, which has a very slow cooling rate other than the Stelmor method, or controlled cooling in a rolling mill equipped with a control rolling facility, can suppress the formation of low temperature tissue by miniaturizing austenite particles. However, this has the disadvantage that it is possible to completely replace the existing Stallmore equipment.

The present invention is to suppress the formation of low-temperature structure (bainite + martensite) in a conventional cooling system without introducing new equipment such as EDC equipment or controlled rolling equipment, high silicon that can be secured as a homogeneous and fine ferrite + pearlite structure It is an object of the present invention to provide a method for producing an added medium carbon steel wire rod.

Wire rod manufacturing method of the present invention for achieving the above object, by weight% carbon 0.40-0.60%, 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, 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% selected from the group consisting of one or more selected from the group consisting of the remaining Fe and other impurities, heated at 950 ~ 1150 ℃ hot-rolled re-rolling, water immediately after rolling Rapid cooling by winding to 760-790 ° C, followed by air cooling at 1.4 ± 0.4 ° C / sec up to 670 ± 30 ° C and slow cooling at 0.6 ± 0.3 ° C / sec up to 595 ± 65 ° C. It is then configured to include cooling to the ferrite + perlite structure by air cooling.

Hereinafter, the present invention will be described in detail.

The present invention is characterized in that the high-silicon-added medium carbon steel is manufactured from wire rods without the occurrence of low-temperature structure by controlled cooling in the wire rod rolling process. The steel component system of the high silicon-added medium carbon steel which is the target steel grade of this invention is demonstrated first, and then the method of wire-rolling this steel is demonstrated.

[High-silicon added medium carbon steel]

The content of carbon (C) is preferably limited to 0.40-0.60%. If the carbon content is less than 0.40%, the total amount of retained austenite, the shape and size of the retained austenite, and the mechanical and thermal stability in the ferritic + residual austenite composite tissue for the production of ferritic + residual austenite composite steel This is because it is difficult to secure sufficient tensile strength and yield strength as high strength bolt steel. If it exceeds 0.60%, abnormal region ferrite is hardly generated, and after heat treatment, proper cross-sectional reduction rate, elongation rate and impact toughness deteriorate, and segregation and surface flaws occur in wire rod manufacturing, deep decarburization when charging furnace, and bolting It affects permanent deformation and static fatigue characteristics, shape and size of microcomposite, transformation time to secure ferrite + residual austenite complex, carbon concentration in residual austenite, and interfacial tool distribution.

The content of silicon (Si) is preferably limited 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 to obtain an adequate amount of retained austenite. It is difficult to secure, and also affects delayed fracture resistance, surface corrosion characteristics, impact toughness, bainite structure, permanent deformation during bolting, and surface ferrite decarburization in wire furnace for controlling wire decarburization. This is because decarburization is intensified due to difficulty in securing the uniformity and proper thickness of the layer, and it is difficult to control the surface scale characteristics due to the increase in the hardenability during the wire cooling. In the case of more than 4.0%, the above-mentioned effect is not preferable because it saturates and affects hardenability, bainite structure, impact toughness, fatigue characteristics, etc., and it is not preferable for the production of bloom or billet for wire rod manufacturing. This is because the quality characteristics in the final product are lowered due to the inhomogeneity of the microstructure due to the segregation of silicon, and the thickness of the surface ferrite layer is increased during heat treatment, which makes it difficult to control the homogeneous surface decarburization.

Manganese (Mn) is an element that forms a solid solution to form a solid solution to strengthen the solid solution, and is very useful for high-strength bolt characteristics. Therefore, considering the strength of the base material, hardenability during heat treatment, stress relaxation, and harmful effects of segregation generation, It is preferable to set it as 0.1-0.8%. 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. Because it's crazy. That is, macro sedimentation and micro segregation are easy to occur depending on the segregation mechanism during steel solidification. Manganese segregation promotes segregation due to relatively low diffusion coefficient compared to other elements, and the improvement of hardenability is due to core martensite). In addition, when manganese is less than 0.1%, the formation of segregation zones due to manganese segregation is hardly achieved, but the effect of stress relaxation is not expected due to the lack of solid solution strengthening effect.

The content of chromium (Cr) is preferably 0.1-0.8%. 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 hardenability improvement is difficult to expect. In addition, if it exceeds 0.8%, it is not preferable because the transformation time of the ferrite + residual austenite composite structure becomes long during isothermal heat treatment, and it is difficult to generate a surface titration ferrite layer when charging the wire rod for controlling the wire decarburization layer. This affects control.

The content of phosphorus (P) and sulfur (S) is preferably 0.01% or less. 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 0.005-0.01%, because it is difficult to form vanadium and niobium-based nitrides that act as non-diffusing water trap sites at 0.005%, and when it exceeds 0.01%, This is because the effect is saturated.

The content of oxygen (O) is preferably less than 0.005%, because when the oxygen content is less than 0.005% coarse oxide-based non-metallic inclusions are easily formed and fatigue life is reduced.

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%, it is difficult to expect the effect of improving the delayed fracture stability due to the incomplete formation of the surface thickening layer, and the heat treatment time is increased during the spheroidization or graphitization treatment to improve decarburization control, toughness and cold formability. This is because there is no improvement effect of cold forming during bolt forming. This is because when the content of nickel exceeds 2.0%, the effect is saturated and affects the appropriate amount, size and shape of the amount of residual 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 0.001-0.003%. If the boron content is less than 0.001%, the grain boundary segregation of boron atoms is insufficient during heat treatment, so the grain boundary strength improvement is not large, and the graphitization promoting effect is insufficient during the graphitization treatment for the improvement of cold formability. In addition, when the content of boron exceeds 0.003%, the effect is saturated, and rather, precipitation of boron nitride at the grain boundary causes a drop in grain boundary strength.

Vanadium (V) or niobium (Nb) is an element that improves delayed fracture resistance and stress relaxation resistance, and its content is preferably 0.01-0.5%. 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 carbide which is not dissolved in the base metal during austenite heat treatment increases, thereby increasing the amount of nonmetallic inclusions. This is because it causes a decrease in fatigue characteristics.

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 It is not desirable because it has a long length.

The content of titanium (Ti) is preferably made 0.01-0.2%. If the titanium content is less than 0.01%, the effect of refining austenite grains is insufficient, and the precipitation distribution of titanium-based carbon and nitrides in the grain boundaries effective for delayed fracture resistance is insufficient. In addition, when it exceeds 0.2%, the effect is saturated, and coarse titanium-based carbon and nitride are formed to affect the mechanical properties.

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

[Manufacturing method of wire rod]

The steel sheet (billet) formed as described above is wire-rolled, the heating temperature at this time is preferably set to 950 ~ 1150C. This is to control the surface quality (decarburization) on the final product after wire rod manufacturing. If the heating temperature of wire rod is lower than 950 ℃, there is a problem in controlling the proper thickness of the billet surface ferrite layer for decarburization control. It is not easy to re-use the precipitated vanadium- or niobium-based precipitates, and the workability is poor due to the overload during rolling due to the increase in hot deformation resistance, and when it exceeds 1150 ° C, a uniform ferrite layer for decarburization control is provided. This is because it cannot be deposited on the surface. In other words, in order to precipitate the surface ferrite layer having a very low carbon solubility, the surface ferrite layer should remain at the heating and holding temperature, but if the heating temperature is higher than 1150 ° C, the ferrite layer on the surface will be austenite. Because of the metamorphosis, the decarburization rate increases rapidly, which intensifies the surface decarburization. As for the heating time at this time, 1-3 hours are preferable.

Immediately after the high-speed hot rolling in the wire rod state as described above, it is rapidly cooled to 760 to 790 ° C by water spraying and wound up. At this time, the wire diameter of the wire rod is preferably manufactured to a diameter of 30mm or less. The temperature range of the austenite single phase zone can maintain a fine austenite grain and the transition start temperature and time in the continuous cooling Transformation (CCT) curve usually moves in a high temperature and a short time direction compared to the cooling start temperature of 850 ℃ It is advantageous to suppress the possibility of the low temperature tissue caused by the lowering of the curing ability.

After rapid cooling and winding as described above, it is cooled to a cooling rate of 1.4 ± 0.4 ° C / sec up to 670 ± 30 ° C. Here, the cooling temperature is ± 30 ℃ based on 670 ℃ and the cooling rate is within ± 0.4 ℃ / sec based on 1.4 ℃ sec. It is set in consideration of the difference of cooling degree between and overlapping part. The conditions of the cooling temperature and the cooling rate are to secure the fine ferrite + perlite structure with good freshness and correspond to an upward position of noise on the CCT curve. However, when cooling to a temperature higher than 670 + 30 ℃ at a cooling rate of 1.4 ± 0.4 ℃ / sec, the appropriate transformation time in the cooling zone is insufficient, which is highly likely to cause low temperature tissue and also lower than 670-30 ℃. Even when cooled to temperature, the possibility of low temperature tissue is very high.

Cooling as above and then to 595 ± 65 ℃ is preferably cooled at a cooling rate of 0.6 ± 0.3 ℃ / sec. If the cooling stop temperature is higher than 595 + 65 ℃, the possibility of low temperature structure during air cooling due to the presence of unaffected austenite is high, and if the cooling stop temperature is lower than 595-65 ℃ 0.6 slow cooling rate of the present invention It is impossible to secure the proper structure of ferrite + pearlite structure because the cooling rate is beyond ± 0.3 ° C / sec. In addition, if the cooling rate up to 595 ± 65 ℃, the cooling stop temperature is faster than 0.6 + 0.3 ℃ / sec, the possibility of low temperature structure is high, and if it is less than 0.6-0.3 ℃ / sec, it is due to the limitation of slow cooling equipment. This is because it is difficult to secure an appropriate cooling temperature range of 595 ± 65 ℃, resulting in low temperature tissue.

As described above, after cooling slowly to 595 ± 65 ° C., air cooling is performed because the change in cooling rate has no effect on the tissue in the state where the transformation is completed.

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

EXAMPLE

The billet (160 × 160mm) having the composition shown in Table 1 below was maintained at 1100 ° C. for 2 hours, and then rolled at high speed to make a wire having a diameter of 14 mm, and the wires were manufactured by different cooling conditions of the invention and the comparative material.

Steel grade C Si Mn Cr V Ni Mo Ti W B P S N ₂ One 0.45 3.03 0.29 0.58 0.05 - - - - - 0.005 0.004 0.008 2 0.40 3.42 0.31 0.79 0.2 - - 0.01 - 0.0013 0.006 0.005 0.014 3 0.60 2.99 0.32 0.33 0.05 0.54 - - 0.02 - 0.007 0.009 0.007 4 0.45 2.0 0.77 0.51 0.11 - 0.2 0.03 - - 0.006 0.008 0.009 5 0.44 3.96 0.23 0.27 0.06 - - - 0.2 0.0015 0.009 0.008 0.008 6 0.53 3.01 0.35 0.55 - - 0.05 0.05 0.07 0.0010 0.004 0.009 0.004 7 0.58 2.56 0.80 0.29 - 1.10 0.13 0.10 - - 0.005 0.006 0.005 Grade 1-7 contains less than 0.005% O. Grade 7 contains 0.01% Nb and Cu, respectively.

Inventive Example (1-9) of Table 2 is hot-rolled and rapidly cooled by water spraying in the range of 760-900 ° C, and then cooled at a cooling rate in the range of 1.0-1.8 ° C / sec to the range of 620-700 ° C. After cooling slowly to the cooling rate range of 0.3-1.8 ℃ / sec to 530-660 ℃, it is cooled to room temperature. In addition, Comparative Example (1-10) was hot rolled and rapidly cooled by water spraying in the range of 790-900 ° C, then cooled to a cooling rate range of 2.0-5.0 ° C / sec to 620-830 ° C, and 530- After cooling slowly to 0.3-2.0 degreeC / sec to 730 degreeC, it cooled to room temperature. The low temperature tissue generation rate of the wire rods prepared as described above was measured using an image analyzer, and the test surface was based on 300 mm ² .

division Steel grade used Cooling start temperature (winding temperature, ℃) Primary cooling temperature (℃) Cooling rate up to the first cooling temperature (℃ / sec) Secondary cooling temperature (℃) Cooling rate up to secondary cooling temperature (℃ / sec) Low temperature tissue formation rate Inventive Example 1 Steel grade 1 780 700 1.0 660 0.3 0 Inventive Example 2 Steel grade 1 780 670 1.4 595 0.6 0 Inventive Example 3 Steel grade 1 780 640 1.8 530 0.9 0 Inventive Example 4 Steel grade 1 760 620 1.6 560 0.8 0 Comparative Example 1 Steel grade 1 830 781 2.3 659 1.0 20 Comparative Example 2 Steel grade 1 850 817 3.3 703 0.6 15 Comparative Example 3 Steel grade 1 900 830 5.0 730 1.3 30 Inventive Example 5 Steel grade 2 790 700 1.4 700 0.4 0 Inventive Example 6 Steel grade 3 760 670 1.8 640 0.7 0 Inventive Example 7 Steel grade 4 790 640 1.5 660 0.7 0 Inventive Example 8 Steel grade 5 760 620 1.7 680 0.9 0 Inventive Example 9 Steel grade 7 790 761 1.8 660 0.6 0 Comparative Example 4 Steel grade 2 780 700 3.0 640 0.3 15 Comparative Example 5 Steel grade 3 780 670 5.0 540 1.2 20 Comparative Example 6 Steel grade 3 780 640 2.0 530 1.4 10 Comparative Example 7 Steel grade 4 760 620 2.6 560 2.0 18 Comparative Example 8 Steel grade 5 790 700 2.9 650 1.5 15 Comparative Example 9 Steel grade 6 790 570 2.5 600 1.6 10 Comparative Example 10 Steel grade 7 790 700 2.2 660 1.3 10

As shown in Table 2, in the inventive example (1-9) prepared by the methods of the present invention, the microstructure in the wire rod state was fine ferrite + having no low-temperature tissue (bainite or martensite) in the matrix. While it is possible to secure the pearlite structure, in the comparative example (1-10), when the phase fraction of the low temperature tissue is distributed in the range of 10-30%, the low temperature tissue is very effective in the wire cooling control method of the present invention. It was well understood that it was controlled.

As described above, the present invention suppresses the formation of low-temperature structure (bainite + martensite) by controlling and cooling only the wire rod in an ordinary cooling facility without using austenitic particle refining method in an EDC method or a controlled rolling facility. It is possible to provide a high silicon-added medium-carbon steel wire composed of a homogeneous and fine ferrite + perlite structure.

Claims (1)

By weight% carbon 0.40-0.60%, 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 0.01-0.2% Billet composed of one or two or more selected from the group consisting of, the remaining Fe and other impurities are heated at 950 ~ 1150 ℃ hot-rolled re-rolling, immediately after rolling and wound by rapid cooling to 760-790 ℃ by water spraying Air cooling at 1.4 ± 0.4 ℃ / sec up to 670 ± 30 ℃ and slow cooling at 0.6 ± 0.3 ℃ / sec up to 595 ± 65 ℃, followed by air cooling to secure ferrite + perlite structure. A method for producing a high silicon-added medium-carbon steel wire without low temperature structure.