KR20010060754A - Method for manufacturing high carbon wire rod containing high silicon to reduce decarburization depth of its surface - Google Patents

Abstract

PURPOSE: A method for manufacturing a wire rod is provided which reduces the surface decarburization layer generated according to the increase of addition amount of silicon by forming a Ferritic layer having a low solid solution of carbon on the surface of billet. CONSTITUTION: The method for manufacturing a high silicon added high carbon steel wire rod having the small surface depth of decarburization comprises the processes of obtaining billet on the surface of which ferritic decarburization layer is formed by reheating and rolling a bloom containing 0.65 to 1.50 wt.% of carbon, 2.0 to 4.0 wt.% of silicon, 0.1 to 0.8 wt.% of manganese, 0.1 to 0.8 wt.% of chromium, 0.01 wt.% or less of phosphorus, 0.01 wt.% or less of sulphur, 0.005 to 0.01 wt.% of nitrogen, 0.005 wt.% or less of oxygen, one or more elements selected from the group consisting of 0.3 to 2.0 wt.% of nickel, 0.001 to 0.003 wt.% of boron, 0.01 to 0.5 wt.% of vanadium, 0.01 to 0.5 wt.% of niobium, 0.01 to 0.5 wt.% of molybdenum, 0.01 to 0.2 wt.% of titanium, 0.01 to 0.5 wt.% of tungsten and 0.01 to 0.2 wt.% of copper, and a balance of Fe and other impurities within the temperature range of 1260±30 deg.C for 100±20 minutes; and wire rod rolling the billet after heating the billet to a heating temperature of 1100±50 deg.C in an average heating speed of 10 deg.C/min or more and maintaining the billet for 45± 15 minutes.

Description

{Method for manufacturing high carbon wire rod containing high silicon to reduce decarburization depth of its surface}

The present invention relates to a method for manufacturing wire rods processed by bolts and the like, and more particularly, to form a ferrite layer having a low carbon employment rate on the surface of a billet to reduce the surface decarburization layer generated as the amount of silicon added is high. The present invention provides a method for manufacturing a wire rod.

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 breakdown', in which cracks are advanced by hydrogen under constant load.

For example, the bolt is a problem that the delayed fracture resistance is deteriorated, it is currently impossible to use more than 130kg / mm ² grade tensile strength is the situation that its 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 steel made of high-silicon-added high carbon having a microstructure of ferrite + residual austenite, which has no possibility of precipitation of Fe-based precipitates at grain boundaries.

However, when silicon steel is manufactured from high-silicon-added high carbon steel as a wire, when reheating in the wire rolling process, silicon increases the activity of carbon in the material and greatly reduces the diffusion coefficient of carbon. As a result, the carbon deactivation rate increases at the surface due to the increase of carbon activity during reheating in the wire rod rolling process. On the contrary, the carbon supply is not smoothly supplied to the surface due to the reduction of the diffusion coefficient at the center, thereby promoting the concentration of carbon at the surface. As the depth of the decarburized layer deepens on the surface, the tightening force decreases in the bolt and the fatigue property in the spring becomes poor. As such, it is well known that the surface decarburization of the high silicon-added steel is because the surface decarburization rate is much faster than that of the low silicon-added steel. Thus, when manufacturing a high silicon-added steel wire, many problems in use may occur when the decarburization control of the material is not appropriate.

Representative examples of decarburization control methods known so far include Korean Patent Nos. 92-24974, 92-24163, 92-24161, Japanese Patent Publications (Pyeong) 2-301514, (Pyeong) 1-31960, (Small) 63 And -216591. These are methods of controlling decarburization by lowering the content of silicon, or adding alloying elements such as lead and tin. However, it is not possible to lower the silicon content in steel grades in which high silicon addition is inevitable for high strength, and when alloying elements such as lead and tin are added, mechanical properties such as impact toughness become poor.

The present invention was devised in the course of research to prevent the decarburization layer on the surface of the high-silicon-added high carbon steel wire heating process, by forming a ferrite decarburization layer of low carbon utilization on the surface of the billet before heating the wire The present invention provides a method for producing a wire rod which can reduce the thickness of the surface decarburization layer by oxidizing the ferrite layer to significantly reduce the decarburization speed of the billet surface.

Wire rod manufacturing method of the present invention for achieving the above object, in weight%, carbon: 0.65-1.50%, silicon: 2.0-4.0%, manganese: 0.1-0.8%, chromium: 0.1-0.8%, phosphorus: 0.01% Sulfur: 0.01% or less, Nitrogen: 0.005 to 0.01%, Oxygen: 0.005% or less, Nickel: 0.3-2.0%, Boron: 0.001 to 0.003%, Vanadium: 0.01 to 0.5%, Niobium: 0.01 to 0.5% , Molybdenum: 0.01% to 0.5%, titanium: 0.01% to 0.2%, tungsten: 0.01% to 0.5%, copper: 0.01% to 0.2%, selected from the group consisting of remaining Fe and other impurities The heated bloom is reheated in the range of 1260 ± 30 ° C within the range of 100 ± 20 minutes and rolled to obtain a billet having a ferrite layer formed on the surface, and the billet is heated at an average heating rate of 10 ° C / min or more at 1100 ± 50 ° C. The temperature is increased to 45 ± 15 minutes, and the wire is rolled.

Hereinafter, the present invention will be described in detail.

The present inventors conducted a multi-angle study to reduce the decarburization of high-silicon-added high carbon bolted steel wire, and found that if the ferrite layer was uniformly distributed on the surface of the billet before the wire heating process of the high-silicon-added high carbon steel, the initial billet surface Due to the very low carbon solubility in the surface ferrite layer, the diffusion of carbon atoms through the ferrite layer is very slow, which can significantly reduce the decarburization rate of the billet surface during heating by wire rod heating. The oxidized (scale out) during the billet heating is gradually reduced to obtain a result that can significantly improve the decarburized layer of the billet surface when extracted by wire heating to complete the present invention. The high silicon-added high carbon steel which is the object of the present invention will be described first, and then a method of manufacturing the wire while controlling the decarburization of the steel will be described.

[High silicon-added high carbon steel]

In the steel of the present invention, the content of carbon (C) is preferably 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 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 composite structure and the appropriate amount of retained austenite. It also affects delayed fracture resistance, surface corrosion characteristics, impact toughness, permanent deformation during bolting, and ensures uniformity and proper thickness of the surface ferrite decarburized layer in the wire heating furnace for wire decarburization control. It is difficult to further decarburize, and it is difficult to control surface scale characteristics due to increased hardenability during wire 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.

Manganese (Mn) is an element that forms solid solids in the matrix and strengthens solid solution, which is very useful for the characteristics of high-strength bolts. It is preferable to set it as 0.1-0.8% in consideration of these. In case of less than 0.1% manganese segregation, segregation zone due to manganese segregation is hardly formed. However, due to insufficient solidification effect, hardenability and improvement of permanent deformation resistance are insufficient. This is because the local quenchability due to the simple stone increases and the anisotropy of the tissue deepens due to the formation of the segregation zone, resulting in the inhomogeneity of the tissue and deteriorating the bolt characteristics.

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 transformation time of the ferrite + residual austenite composite structure becomes longer during isothermal heat treatment, and it is difficult to generate a surface titration ferrite layer when charging the wire rod for controlling the wire decarburization layer. 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. 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 0.005% or less. This is because when the content of oxygen exceeds 0.005%, coarse oxide-based nonmetallic inclusions are easily formed and fatigue life is reduced.

Vanadium (V) or niobium (Nb) is an element that improves delayed fracture resistance and stress relaxation resistance, and 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 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 formation of the surface thickening layer 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. 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 effect of improving grain boundary strength 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 graphitization treatment for improving 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 (Ti) is preferably made 0.01-0.2%. If the titanium content is less than 0.01%, the austenite grain refining effect is insufficient, and the precipitation distribution of titanium-based carbon and nitride in the grain boundary effective for delayed fracture resistance is insufficient, so that the improvement effect is difficult to be expected. If exceeded, the effect of saturation is saturated and coarse titanium-based carbon and nitride are formed to affect the mechanical properties.

[Manufacturing method of wire rod]

In the steel formed as described above, a decarburized layer is formed on the surface of the heating process for manufacturing the wire rod. In order to prevent this, it is important to form a ferrite decarburized layer on the surface of the billet and to control heating conditions for the wire rod rolling. .

First, the bloom is reheated in the range of 1260 ± 30 ° C. within a range of 100 ± 20 minutes and rolled into steel to form a ferrite layer on the surface of the billet, wherein the thickness of the ferrite layer is preferably 0.05 to 0.20 mm. . If the heating temperature of BLUM is less than 1230 ℃, the thickness of ferrite layer becomes larger than necessary in the steel piece heating furnace. If the heating temperature is over 1290 ℃, the oxidation rate is increased so that it is difficult to secure the proper ferrite layer thickness on the surface. This is because the amount of oxidation is increased and the surface flaw is more likely to occur during rolling. If the heating holding time is less than 80 minutes, the thickness of the ferrite layer cannot be secured due to lack of time, and if the heating time is over 120 minutes, the effect is saturated and the surface quality is deteriorated due to the high oxidation amount.

In the present invention, as described above, the ferrite decarburized layer is preferably formed at 0.05 to 0.20 mm on the surface of the billet. The reason is that when the ferrite layer is less than 0.05 mm, most of the ferrite layer is oxidized during heating of the billet, so that the decarburization improvement effect is difficult due to the presence of the ferrite layer, and when the thickness of the ferrite layer exceeds 0.20 mm, the heating time is too long. It is uneconomical to lose.

As described above, the billet having the ferrite layer formed on the surface is wire rolled. First, a billet is heated up at 15 degree-C / min or more. This is because when the temperature increase rate is less than 15 ° C./minute, the oxide layer (scale layer) on the surface of the billet becomes thicker, and as the oxide layer grows, pores of the pores are generated in the oxide layer, thereby rapidly decreasing the oxidation rate. Reduction of the oxidation rate is undesirable because it makes it difficult to control the thickness of the ferrite decarburized layer.

The temperature is raised to a range of 1100 ± 50 ° C. at the same rate of temperature increase, and then maintained at this temperature range for 45 ± 15 minutes. This is because it is difficult to control the thickness of the billet surface ferrite layer for the decarburization control when the heating temperature is less than 1050 ° C, and it is not easy to re-use the coarse precipitated vanadium- or niobium-based precipitates during the manufacture of the billet, and to increase the thermal strain resistance. This is because the workability becomes poor due to the overload during furnace rolling. When the heating temperature exceeds 1150 ° C., it is not possible to maintain a uniform ferrite layer for decarburization control. 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 maintaining temperature, but if the heating temperature exceeds 1150 ℃, the ferrite layer on the surface will be austenite. Because of the metamorphosis, the decarburization rate increases rapidly, which intensifies the surface decarburization.

As described above, the billet is heated and wired, but the decarburized area of the billet extracted from the heating furnace is the same as the decarburized area of the wire after rolling. Accordingly, in the present invention, the wire rod of the wire rod is preferably rolled up to 30 mm in diameter in view of such a point.

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

EXAMPLE

In accordance with the present invention, a high-silicon-added high carbon broth having a component system as shown in Table 1 below was rolled and rolled in accordance with the present invention to the invention example (1-14). It was set as the example (1-5).

Inventive Example (1-14) is reheated and rolled into a range of 100 ± 20 minutes in the range of 1260 ± 30 ℃, and then heated to a range of 1100 ± 50 ℃ at a heating rate of 15 ℃ / min on average, After maintaining for 30-40 minutes, the wire rod was rolled to prepare a wire rod having a diameter of 12 mm. The comparative example (1-5) is reheated and rolled in a steel strip for a time of less than 80 minutes or more than 120 minutes at a temperature of less than 1230 ℃ or more than 1290 ℃, the billet obtained here 1050 at a heating rate of 8 ~ 15 ℃ / min The temperature was lowered to less than or equal to or higher than 1150 ° C, and the wire was rolled by maintaining the temperature for 30 minutes at this temperature section to prepare a wire having a diameter of 12 mm.

Both the invention material and the comparative material were rolled by high speed wire, then rapidly cooled by water spraying at 950 ° C., wound up, and slowly cooled to 1.0 C / sec up to 600 ° C., followed by air cooling. The surface decarburized layer depth of the inventive material and the comparative material prepared as described above was measured by the KS standard (KD D 0216). According to this standard, an optical microscope observation method and a microhardness measuring method are proposed, but the microhardness measuring method was used in the present Example. The decarburized layer depth measured as described above is shown in Table 2. The measurement position was measured at 8 equal sections of wire rod section, and the measured value was based on the average value.

division C Si Mn Cr Ni B V Mo Ti W P S N Steel grade 1 0.81 2.93 0.33 0.49 - - 0.04 - - - 0.007 0.009 0.006 Steel grade 2 0.68 3.54 0.35 0.74 - 0.0010 0.19 - 0.01 - 0.009 0.006 0.012 Steel grade 3 0.90 3.04 0.29 0.38 0.66 - 0.06 - - 0.03 0.004 0.008 0.008 Steel grade 4 0.83 2.09 0.71 0.55 - - 0.12 0.25 0.03 - 0.005 0.004 0.011 Steel grade 5 0.82 3.92 0.32 0.32 - 0.0019 0.05 - - 0.18 0.006 0.004 0.008 Steel grade 6 1.21 3.11 0.30 0.56 - 0.0013 - 0.04 0.05 0.09 0.007 0.006 0.005 Steel grade 7 1.42 2.61 0.79 0.33 1.10 - - 0.10 0.10 - 0.009 0.005 0.005 Grades 1 to 7 contain less than 0.005% of O. Grade 7 contains Nb: 0.01% and Cu: 0.01%.

division Target steel grade Gangbyeon Heating Furnace Billet Ferrite Decarburization Depth (mm) Wire Rod Furnace Wire rod total decarburization depth (mm) Heating temperature (℃) Heating time (min) Temperature increase rate (℃ / min) Heating holding temperature (℃) Heating holding time (min) Inventive Example 1 Steel grade 1 1230 100 0.06 15 1050 40 0.04 Inventive Example 2 1260 100 0.13 15 1100 30 0.04 Inventive Example 3 1290 100 0.08 15 1100 30 0.05 Inventive Example 4 1260 80 0.10 15 1100 30 0.06 Inventive Example 5 1260 120 0.10 15 1100 30 0.07 Inventive Example 6 1260 100 0.16 20 1100 30 0.05 Inventive Example 7 1260 100 0.16 15 1050 40 0.05 Inventive Example 8 1260 100 0.16 15 1150 30 0.06 Inventive Example 9 Steel grade 2 1260 100 0.14 15 1050 30 0.07 Inventive Example 10 Steel grade 3 1260 100 0.15 15 1050 30 0.06 Inventive Example 11 Steel grade 4 1260 100 0.12 15 1050 30 0.06 Inventive Example 12 Steel grade 5 1260 100 0.17 15 1050 30 0.05 Inventive Example 13 Steel grade 6 1260 100 0.13 15 1050 30 0.04 Inventive Example 14 Steel grade 7 1260 100 0.09 15 1050 30 0.06 Comparative Example 1 Steel grade 1 1200 100 0.35 15 1050 30 0.12 Comparative Example 2 1320 100 0.01 15 1050 30 0.14 Comparative Example 3 1260 150 0.03 15 1000 30 0.19 Comparative Example 4 1260 100 0.16 15 1200 30 0.20 Comparative Example 5 1260 100 0.16 8 950 30 0.25

As shown in Table 2, the surface decarburization depth of the wire rods prepared according to the present invention showed a range of 0.04-0.07 mm, whereas in the case of Comparative Example (1-5), the surface decarburization depth was 0.12-0.25 mm. It can be seen that it is very effective for the improvement.

As described above, the present invention has a useful effect of providing a high-silicon-added high carbon steel wire that can be used in bolts and the like by distributing an appropriate ferrite layer in the billet to suppress decarburization in the wire furnace.

Claims (2)

By weight%, 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, nickel: 0.3-2.0%, boron: 0.001 to 0.003%, vanadium: 0.01 to 0.5%, niobium: 0.01 to 0.5%, molybdenum: 0.01 to 0.5%, titanium: 0.01 to 0.2% , Tungsten: 0.01% to 0.5%, copper: 0.01% to 0.2%, containing 1 or 2 or more selected from the group consisting of the remaining Fe and other impurities in the range of 1260 ± 30 ℃ 100 ± 20 minutes Reheated and rolled in a range of to obtain a billet with a ferrite decarburized layer formed on the surface.The billet was heated to a heating temperature of 1100 ± 50 ° C at a heating rate of 10 ° C / min or more and maintained for 45 ± 15 minutes. A method for producing a high silicon-added high carbon steel wire having a low surface decarburization depth. The method of manufacturing a high silicon-added high carbon steel wire having a low surface decarburization depth according to claim 1, wherein the ferrite layer decarburization layer of the billet has a thickness of 0.05-0.20 mm from the surface.