KR20030055521A - Method for high carbon wire rod containing high silicon to reducing decarburization depth of its surface - Google Patents

Abstract

PURPOSE: A method for manufacturing high Si added high carbon wire rod for bolt by forming decarburized ferritic layer is provided. CONSTITUTION: The method includes the steps of heating a steel bloom comprising C 0.65 to 1.50 wt.%, Si 2.0 to 4.0 wt.%, Mn 0.1 to 0.8 wt.%, 0.01 wt.% or less of P and S, N 0.002 to 0.01 wt.%, 0.002 wt.% or less of O, one or more than two elements selected from the group consisting of Ni 0.3 to 2.0 wt.%, B 0.001 to 0.003 wt.%, V 0.01 to 0.5 wt.%, Nb 0.01 to 0.5 wt.%, Mo 0.01 to 0.5 wt.%, Ti 0.01 to 0.2 wt.%, W 0.01 to 0.5 wt.% and Cu 0.01 to 0.2 wt.%, a balance of Fe and incidental impurities at 1260±30°C for 100±20 min; hot rolling the steel bloom, wherein ferrite decarburized layer is formed on the surface of the steel billet; heating the steel billet to 1100±50°C at a rate of greater than 10°C/min; holding the steel billet at 1100±50°C for 45±15 min; and hot rolling the steel billet into wire rod.

Description

Method for producing high silicon wire rod with low silicon decarburization depth {Method for high carbon wire rod containing high silicon to reducing decarburization depth of its surface}

The present invention relates to a method for manufacturing a wire rod that is processed by bolts and the like, and more particularly, in the steel that can be graphitized for the purpose of improving cold formability, by forming a ferrite layer having a low carbon utilization on the surface of a billet. It is to provide a method for producing a wire rod that can reduce the surface decarburization layer generated by the high amount of silicon added.

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 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. 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 grain boundaries act as trapped sites of hydrogen and degrade the strength of grain boundaries. It is most important to suppress the distribution of Fe-based precipitates to be distributed to the maximum.

In addition, the shape of the various bolts are usually manufactured by cold forming, high-strength material is required to soften the material heat treatment before the cold molding high strength, it is desirable to secure the cold-molding tensile strength of 60kg / mm ² or less. This is to suppress the increase of the die wear rate during cold forming to the maximum. Domestic steel structure fastening bolts are currently capable of cold forming bolts under tensile strength of 60kg / mm ² or less. Therefore, in order to use high-strength bolt material, it is necessary to secure not only excellent delayed fracture resistance but also cold forming required for bolt manufacturing.

In cold forming high strength bolts, the present inventors can secure excellent cold formability in terms of material tensile strength or surface hardness with or without addition of alloying elements when using graphitized structure and improve delay fracture resistance. In order to develop a steel made of high-silicon-added heavy carbon, which has a fine composite structure with no possibility of precipitation of Fe-based precipitates in grains,

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 for preventing decarburization layer on the surface in the wire heating process of high-silicon-added high carbon steel which can be graphitized for the purpose of improving the cold forming property, and the carbon on the surface of the billet before the wire heating. The present invention provides a method for producing a wire rod that can reduce the thickness of the surface decarburization layer by forming a ferrite decarburization layer having a low solubility and 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%, phosphorus and sulfur 0.01% or less, nitrogen 0.002-0.01%, Oxygen 0.002% 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 0.01-0.2%, tungsten 0.01 Bloom, composed of one or two or more selected from the group of -0.5%, 0.01-0.2% copper, and the remaining Fe and other impurities, is reheated and rolled into a range of 100 ± 20 minutes in the range of 1260 ± 30 ℃ A billet on which a ferrite decarburized layer was formed was obtained, and the billet was heated to a heating temperature of 1100 ± 50 ° C. at a heating rate of 10 ° C./min or more, maintained for 45 ± 15 minutes and 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 wires. Because the surface ferrite layer distributed at, has a very low carbon solubility, diffusion of carbon atoms through the ferrite layer is very slow, which leads to a significant reduction in the decarburization rate of the billet surface during heating. . In addition, the ferrite layer is oxidized (scale out) during the heating of the billet and is gradually reduced, thereby obtaining a result of remarkably improving the decarburized layer on the surface of the billet when extracted by wire heating, thereby completing 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 composition]

Carbon (C): 0.65-1.50%

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 composite structure after heat treatment for the production of ferritic + bainite composite steel, and it is mechanically, thermally stable and has sufficient tensile strength as a high strength bolt steel. This is because it is difficult to secure strength and yield strength. 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 at the same time, the proper shape and size of microcomposite tissues, the transformation time required to secure a composite structure effective in delayed fracture resistance, the carbon concentration in the retained austenite, and the interfacial concentration tool Because it has a bad effect.

Silicon (Si): 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 microstructure is degraded, so that it is difficult to secure a complex structure and an appropriate phase ratio effective for the delayed fracture resistance. Insufficient solid-solution strengthening effect of ferrite makes it difficult to secure strength, and also affects delayed fracture resistance, surface corrosion characteristics, impact toughness, permanent deformation during bolting, and the surface of wire furnace for wire decarburization control. This is because it is difficult to secure uniformity and proper thickness of the ferrite decarburization layer, so that decarburization is intensified, and it is difficult to control surface scale characteristics due to an increase in 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 heat treatment. More preferred silicon component range in the present invention is 2.8-3.3%, isothermal heat treatment time and residual austenite fraction and size, shape, ferrite + residual austenite composite structure for producing bainite structure (ferrite + residual austenite) Strength, high toughness, delayed fracture resistance (diffuse hydrogen content, precipitation control of grain boundary precipitates), surface decarburization, stress relaxation or permanent deformation resistance after bolting, and dynamic and static fatigue characteristics This can be effectively improved.

Manganese (Mn): 0.1% to 0.8%

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.

Phosphorus (P) and sulfur (S): 0.01% or less each

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%.

Nitrogen (N): 0.002-0.01%

This is because when the nitrogen content is less than 0.002%, 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.

Oxygen (O): 0.0020% or less

This is because when the content of oxygen exceeds 0.0020%, coarse oxide-based nonmetallic inclusions are easily formed and fatigue life is reduced.

In the above composition, 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%, One or more selected from the group of 0.01-0.2% copper is added.

Vanadium (V) or niobium (Nb): 0.01 to 0.5% each

Vanadium (V) or niobium (Nb) 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 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, 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 coarse that is not dissolved in the base material during austenite heat treatment. This is because the amount of alloy carbide increases and acts like a non-metallic inclusion, leading to a decrease in fatigue properties.

Nickel (Ni): 0.3 to 2.0%

Nickel (Ni) is an element for promoting graphitization and is an element that improves delayed fracture resistance by forming a nickel enriched layer on the surface during heat treatment to suppress permeation of external hydrogen. 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 (B, B): 0.001% to 0.003%

Boron (boron, B) is a graphitization promoting element in the present invention and is a grain boundary strengthening element for improving quenching and delayed fracture resistance, and the content thereof 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 of boron atoms due to grain boundary segregation during heat treatment is insufficient, and the graphitization promoting effect is insufficient during 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.

Molybdenum (Mo), Tungsten (W): 0.01-0.5% each

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.

Copper (Cu): 0.01-0.2%

If the copper content is less than 0.01%, the effect of promoting graphitization and corrosion resistance is insufficient. If the copper content is more than 0.2%, the improvement effect is saturated, and the melting point is lowered during the grain boundary segregation. This is because surface flaw is more likely to occur due to grain embrittlement during charging, and impact toughness of the final product is lowered.

Titanium: 0.01-0.2%

If the content of titanium is less than 0.01%, the effect of promoting graphitization and miniaturizing 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, so that improvement effect is difficult to expect. This is because when it exceeds 0.2%, the additive effect 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 slab 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 10 degree-C / min or more. This is because when the temperature increase rate is less than 10 ° C./minute, the oxide layer (scale layer) on the surface of the billet becomes thick, and after that, 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 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).

Chemical composition C Si Mn Cr V Ni Mo Ti W B Cu P S N2 O2 Invention steel One 0.80 3.06 0.53 - 0.05 - - - - - 0.03 0.009 0.009 0.004 0.0013 2 0.70 3.44 0.52 - 0.06 - - 0.02 - 0.001 0.04 0.007 0.008 0.004 0.0014 3 0.89 3.13 0.58 - 0.07 0.72 - - 0.06 - 0.12 0.006 0.009 0.005 0.0015 4 0.85 2.25 0.87 - Nb: 0.01 - 0.23 0.04 0.15 - 0.03 0.006 0.009 0.004 0.0016 5 0.83 3.90 0.55 - 0.04 - - - 0.0020 0.03 0.007 0.006 0.005 0.0017 6 1.24 3.22 0.64 - - - 0.05 0.03 0.07 0.0023 0.02 0.007 0.007 0.005 0.0017 7 1.39 2.57 0.82 - - 1.20 0.20 0.09 - - 0.15 0.009 0.008 0.005 0.0018

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 raised to a temperature lower than or equal to 1150 ° C, and the wire rod was rolled by maintaining the temperature for 30 minutes in this temperature section to prepare a wire rod having a diameter of 16 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.

As shown in Table 2, the surface decarburization depth of the wire rods prepared according to the present invention (the invention 1 to 14) shows a range of 0.03-0.06mm, while in the case of Comparative Example (1-5) 0.13-0.24 In mm range, it can be seen that the present invention is very effective in improving surface decarburization.

As described above, the present invention provides a high-silicon-added high carbon steel wire that can be used in bolts and the like by distributing a proper ferrite layer in a billet of steel capable of graphitization for the purpose of improving cold formability and suppressing decarburization in a wire furnace. It has a useful effect.

Claims (2)

By weight, carbon: 0.65-1.50%, silicon 2.0-4.0%, manganese 0.1-0.8%, phosphorus and sulfur 0.01% or less, nitrogen 0.002-0.01%, oxygen 0.002% or less, and 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% Blooms composed of two or more species or the remaining Fe and other impurities are reheated and rolled into a range of 100 ± 20 minutes in the range of 1260 ± 30 ° C to obtain a billet with a ferrite decarburized layer formed on the surface, and the billet is averaged. A method for producing a high-silicon-added high carbon steel wire having a low surface decarburization depth, which is heated at a heating temperature of 1100 ± 50 ° C. at a heating rate of 10 ° C./min or more and maintained for 45 ± 15 minutes. 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.