KR20030055515A - Method for manufacturing high si added medium carbon wire rod by forming decarburized ferritic layer - Google Patents

Abstract

PURPOSE: A method for manufacturing high Si added medium carbon wire rod by forming ferrite decarburized layer is provided. CONSTITUTION: The method includes the steps of reheating a steel bloom comprising C 0.40 to 0.60 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.%, Cu 0.01 to 0.2 wt.%, a balance of Fe and incidental impurities at 1260±30°C for 100±20 min; rolling the steel bloom into a steel billet, on the surface of which ferrite decarburized layer is formed; heating the steel billet up to 1050±50°C at a rate of greater than 10°C/min; holding the steel billet within 1000 to 1100 deg.C for more than 30 min; and rolling the steel billet into wire rod.

Description

Method for manufacturing high silicon-added medium carbon wire rod using ferrite decarburization layer {Method for manufacturing high Si added medium carbon wire rod by forming decarburized ferritic layer}

The present invention relates to a method for manufacturing wire rods processed by bolts and the like, and more particularly, to roll-roll a surface decarburization layer generated by the addition of silicon in a steel capable of graphitization to improve cold forming properties. The present invention relates to a method for producing a wire rod which can reduce and reduce the ferrite decarburization layer having low carbon employment.

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 high 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 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 expected advantages when developing bolt steel with high delay fracture resistance and high strength. In other words, in terms of steel structures, bolt fastening does not require more skilled techniques than welding joints, and considering the advantages of replacing weak welds, firstly, the stability of steel structures can be increased by strengthening bolt fastening force, and second, the number of bolt fastenings is reduced. It is possible to reduce the amount of steel used. In addition, in terms of automotive parts, third, it contributes to the lightening of parts. Fourth, there is an advantage in 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 lowering the delay fracture resistance of the material, the ripple 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 tensile strength of less than 60kg / mm ² 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 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 manufacturing silicon steel as a wire in high-silicon-added high carbon steel, the silicon increases the activity of carbon in the material and greatly reduces the diffusion coefficient of carbon during reheating in the wire rolling process. 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. Most of these methods control decarburization by lowering the silicon content 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 wire heating process of high-silicon-added medium-carbon steel, which can be graphitized for the purpose of improving the cold forming property, the carbon on the surface of the billet in the steel sheet rolling process It is an object of the present invention to provide a method for producing a wire rod which can reduce the thickness of the surface decarburization layer by significantly reducing the decarburization rate on the surface of the billet in the wire rod rolling process by forming a ferrite decarburization layer having a low solubility.

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%, phosphorus and sulfur 0.01% or less, nitrogen 0.002-0.01%, oxygen 0.002 % 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 two or more of the remaining Fe and other impurities in the range of 1260 ± 30 ℃ reheating to 100 ± 20 minutes in the range of 100 ± 20 minutes to the ferrite decarburization The layered billet is obtained, and the billet is heated to a heating temperature of 1050 ± 50 ° C. at a heating rate of 10 ° C./min or more, and maintained for 30 minutes or more.

Hereinafter, the present invention will be described in detail.

The present inventors conducted a multi-angle study to reduce the decarbonization of high silicon-added medium carbon steel wires, and when reheating by controlling the heating temperature and holding time during heating for rolling the high silicon-added high carbon steel bloom into billet, Later it was found that a 0.05-0.10 mm thick ferrite layer could be evenly distributed on the surface of the billet. In addition, the temperature rising through the abnormality (the temperature between the Ac1 transformation point and Ac3 transformation point, the ferrite and austenite phases are mixed to maintain an equilibrium state) during the billet heating (when heating the wire heating) for rolling the billet. When the heating rate is controlled within the range of 15 ° C / min or more, which is a normal condition, the ferrite layer having a very low carbon solubility is uniformly distributed in the surface of the billet with a thickness of 0.05-0.1 mm. Since the diffusion of carbon is very slow, the rate of decarburization can be significantly reduced, and the thickness of the surface ferrite layer can be adjusted according to the degree of oxidation, thereby significantly improving the billet surface decarburization layer during extraction by wire heating. The result was obtained, and the present invention was completed. The high-silicon-added medium-carbon steel, which is the object steel of the present invention, will be described first, and then a method of manufacturing the steel as a wire will be described.

[Composition of High Silicon-added Medium Carbon Steel]

Carbon (C): 0.40-0.65%

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 toughness, limiting its upper limit to 0.01%. Sulfur is a low melting point element, and 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%.

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 quenchability and delayed fracture resistance. 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 improvement of 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]

The steel formed as described above has a high Si content and a decarburized layer is generated in the heating process for manufacturing the wire rod. In order to prevent this, the steel is rolled by controlling the heating conditions in the heating process for rolling the bloom into the bilet. It is important to form a ferrite decarburized layer on the surface of the billet.

First, bloom is rolled by heating the bloom at a temperature of 1260 ± 30 ° C. for 100 ± 20 minutes. If the heating temperature is lower than 1230 ℃, the thickness of the appropriate ferrite layer on the surface of the broom in the steel piece heating furnace increases more than necessary, so it is difficult to obtain decarburization improvement effect.If the heating temperature exceeds 1290 ℃, the oxidation rate increases very much. It is because it is difficult to secure the thickness of the appropriate ferrite layer on the surface, and the amount of oxidation increases, so that there is a high possibility of occurrence of surface defects during rolling of the steel sheet. On the other hand, if the heating holding time is less than 80 minutes, the time required to secure the appropriate thickness of the ferrite layer on the surface of the broom is insufficient, and if it exceeds 120 minutes, the effect is saturated and the surface quality is degraded due to excessive oxidation amount. to be.

The bloom is heated and rolled as described above, wherein the thickness of the ferrite decarburized layer on the surface of the produced billet is preferably 0.05 to 0.1 mm. If the ferrite decarburization layer is less than 0.05 mm, it is difficult to expect the decarburization improvement effect due to the presence of the ferrite layer because most of the ferrite layer is oxidized during the heating of the billet. When the ferrite decarburization layer exceeds 0.10 mm, the heating time may be longer to show the effect of the present invention. Because there is.

As described above, the billet is manufactured and the wire is rolled. In this case, the heating temperature of the wire heating furnace is preferably in the range of 1050 ± 50 ° C. When the heating temperature is less than 1000 ℃, it is difficult to control the thickness of the billet surface ferrite layer for decarburization control, and it is not easy to reconsider 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 is poor due to the overload. In addition, when the heating temperature exceeds 1100 ℃ can not maintain a uniform ferrite layer for decarburization control. That is, in order to precipitate a surface ferrite layer having a very low carbon solubility and to sharply reduce the decarburization reaction, the surface ferrite layer must remain at the heating and holding temperature, but when the heating temperature exceeds 1100 ° C, the surface ferrite layer is transformed into austenite. This is because the decarburization rate increases sharply, which causes deep surface decarburization. The heating holding time is 30 minutes or more because it is difficult to ensure a uniform temperature distribution inside the billet for wire rod rolling in less than 30 minutes.

As described above, the billet is heated to roll the wire. At this time, the decarburization area of the billet extracted from the heating furnace is the same as the decarburization area of the wire after the wire rolling. Therefore, in the present invention, in consideration of this point, it is preferable to roll the wire to a diameter of 30 mm or less.

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 V Ni Mo Ti W B Cu P S N2 O2 Invention steel One 0.44 3.03 0.63 0.05 - - - - - 0.03 0.008 0.009 0.005 0.0013 2 0.43 3.24 0.62 0.06 - - 0.02 - 0.001 0.04 0.008 0.008 0.005 0.0014 3 0.56 3.23 0.68 0.07 0.70 - - 0.04 - 0.15 0.009 0.009 0.005 0.0015 4 0.42 2.35 0.68 Nb: 0.01 - 0.25 0.04 0.12 - 0.05 0.007 0.009 0.005 0.0016 5 0.45 3.99 0.75 0.04 - - - 0.0020 0.04 0.008 0.009 0.005 0.0017 6 0.58 3.12 0.74 - - 0.06 0.03 0.06 0.0020 0.03 0.009 0.008 0.004 0.0017 7 0.57 2.37 0.82 - 1.10 0.23 0.09 - - 0.20 0.009 0.008 0.004 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 1050 ± 50 ℃ at a heating rate of 15 ℃ / min, in this temperature range A wire rod having a diameter of 13 mm was prepared by maintaining the wire for 30 to 40 minutes. 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 obtained billet 1000 at a heating rate of 8 ~ 15 ℃ / min The temperature was lowered to less than or equal to or higher than 1050 ° 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 13 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 wire surface decarburization depth of Inventive Example (1-14) shows a range of 0.03-0.06 mm, whereas in Comparative Example (1-5), the present invention has a wire decarburization range of 0.10-0.25 mm. It can be seen that it is very effective in improving the

As described above, the present invention distributes an appropriate ferrite decarburized layer during heating by steel flake heating for steel that can be graphitized for the purpose of improving cold formability, thereby adding high silicon as an effect of suppressing the decarburization reaction when heating billet by wire rod heating. It can provide medium carbon steel wire rod.

Claims (3)

By weight% carbon 0.40-0.60%, 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 0.01-0.5%, Copper 0.01-0.2% Or reheating the bloom composed of two or more kinds of the remaining Fe and other impurities within the range of 1260 ± 30 ° C. to 100 ± 20 minutes and rolling the steel sheet to obtain a billet having a ferrite decarburized layer formed on the surface, and the billet is averaged at 10 ° C. / A method for producing a high-silicon medium-carbon wire rod using a ferrite decarburization layer comprising heating the wire rod by heating it at a heating temperature of 1050 ± 50 ° C. at a heating rate of at least 30 minutes. The method of claim 1, wherein the ferrite decarburization layer of the billet is formed to a thickness of 0.05 ~ 0.20mm from the surface of the high-silicon-added medium carbon wire using the ferrite decarburization layer. The method of claim 1, wherein the wire is 30mm or less, the method of producing a high-silicon-added medium-carbon wire rod using a ferrite decarburization layer.