KR100398388B1 - Manufacturing method of shaft wire with excellent impact characteristics - Google Patents

Abstract

The present invention relates to a method for manufacturing a wire rod used in a rack bar of a vehicle steering apparatus, and the like, the object of the present invention is to adjust the steel composition and to control the hot wire rolling and cooling conditions appropriately, and to eliminate the quenching-treatment heat treatment. It is another object of the present invention to provide a method for producing a wire rod for a shaft in which impact toughness is improved.

The present invention having such an object is, by weight, C: 0.30-0.60%, Si: 0.10-0.40%, Mn: 0.8-1.5%, Cr: 0.03-1.0%, V: 0.03-0.15%, Ti: 0.010. -0.040%, Al: 0.01-0.10%, P: 0.030% or less, S: 0.020-0.10%, N: 0.005-0.030%, Ni: 0.10-0.30% and the remaining Fe and other inevitable impurities The billet is heated to 950-1100 ℃, hot rolled and wound in coil form, and the 550-750 ℃ section is cooled at a rate of 0.1-1.0 ℃ / sec, followed by cold drawing at a rate of 5.0-30%. The technical gist of the manufacturing method of the wire rod for shafts which is excellent in the impact characteristic including these is made into the technical summary.

This invention has the effect of providing a wire rod of tensile strength 89.1-97kg / mm 2 impact value 6.5-7.5kg.m / cm 2.

Description

Manufacturing method of shaft wire with excellent impact characteristics

The present invention relates to a method for manufacturing a wire rod used in a rack bar of a vehicle steering apparatus, and more particularly, to a method for manufacturing a non-coated steel wire rod which improves impact toughness while eliminating the hardening-treatment heat treatment performed after hot wire rolling. .

Wire rods are used in various mechanical parts and are also applied to the rack bar 10, which is a gear shaft of the automobile steering apparatus shown in FIG. In order to use the wire rod as a rack bar, first, the billet is rolled into hot rod form, and it is hardened and heat treated to improve strength and toughness, followed by cold drawing and cutting process to form gears. Cut the wire. In order to give abrasion resistance to this gear part, a high frequency quenching heat treatment and annealing heat treatment are applied to the rack bar. Usually, the wire used for high frequency quenching heat treatment is called high frequency quenching wire, and cold wire workability (cutting workability and drilling workability), high frequency quenchability and mechanical properties are important. As a high frequency hardening wire rod, S45C, which is a carbon steel for mechanical structure shown in Table 1, is commonly used.

Steel grade Chemical composition (% by weight) C Si Mn P S S45C 0.42-0.48 0.15-0.35 0.60-0.90 Less than 0.030 Less than 0.035

On the other hand, since the mid 70s, non-coated steel wire rods have been developed and widely used in Europe and Japan, which can significantly reduce cost by omitting anneal-thin heat treatment to improve the strength and toughness of wire rods. Non-coated steel wire is a steel that has been developed so that the alloy design is appropriate and high strength tough material is manufactured through controlled rolling and controlled cooling to omit the quenching-thinning heat treatment process following hot forging or cold forging. Representative examples of non-coated steel wire rods include Japanese Patent Laid-Open No. 59-136420 and Japanese Patent Laid-Open No. 7-54040.

The steel disclosed in Japanese Patent Application Laid-Open No. 59-136420 increases the manganese (Mn) content compared with conventional carbon steel for mechanical structure to increase strength without deterioration of toughness, and about 0.1% of vanadium (V), which is a hardening element, of precipitation. After the hot forging, the vanadium carbonitride [V (C, N)] is finely precipitated in the ferrite matrix during the cooling process to increase the strength, thereby eliminating the subsequent hardening and annealing process. This wire rod is suitable to be used as a material to make a specific shape part through hot forging, but it is not suitable for use as a cold work material because of its poor machinability or drilling ability.

In addition, the non-coated steel wire proposed in Japanese Unexamined Patent Publication No. 7-54040 has a very low carbon content to improve cold workability, and a small amount of niobium is added to improve the strength and toughness of the steel through microstructure. will be. This non-coated steel wire is suitable for use in cold drawing and forging materials, but it does not contain free cutting elements, which makes it unsuitable for cutting. It also has low carbon content and poor high frequency quenchability.

So far, the proposed non-coated steel wire can reduce cost by omitting quenching-concern heat treatment, but there is a problem in that it cannot be used as a wire for high frequency quenching due to poor cutting processability or high frequency hardening.

Accordingly, the present inventors have conducted research and experiments to solve the problems of the prior arts as described above and develop wire rods suitable for the rack bar of a vehicle steering apparatus and propose the present invention based on the results.

According to the present invention, steel components are appropriately adjusted and hot wire rolling and cooling conditions are appropriately controlled, thereby eliminating the hardening-treatment heat treatment, and exhibiting the same processing characteristics and mechanical properties as those of tempered steel wires. It is an object of the present invention to provide a method for manufacturing non-coarse steel wire.

1 is a schematic diagram of a vehicle steering apparatus

2 is a microstructure photograph of the invention steel

Wire rod manufacturing method of the present invention for achieving the above object, in weight%, C: 0.30-0.60%, Si: 0.10-0.40%, Mn: 0.8-1.5%, Cr: 0.03-1.0%, V: 0.03- 0.15%, Ti: 0.010-0.040%, Al: 0.01-0.10%, P: 0.030% or less, S: 0.020-0.10%, N: 0.005-0.030%, Ni: 0.10-0.30% and the rest of Fe and other unavoidably The billet composed of the impurity contained was heated to 950-1100 ° C. to be hot rolled and coiled, and then cooled at a rate of 0.1-1.0 ° C./sec at 550-750 ° C., followed by 5.0-30% of It is comprised including cold drawing by a cross-sectional reduction rate.

As described above, the wire drawn by cold drawing is cut and finally subjected to high frequency quenching and annealing to harden the surface to be commercialized.

Hereinafter, the present invention will be described in detail.

The present invention adjusts the steel wire divider to be suitable for the purpose of the wire rod while eliminating the hardening and heat treatment, and in particular, manages the [S] at an appropriate level to secure the cutting property of the wire rod, and an appropriate amount of [Ni] to improve the impact toughness. In addition to controlling the manufacturing process, there is a feature. The present invention is achieved by the organic combination of the steel component and the manufacturing process, which will be described in detail below.

[Steel Ingredients]

The carbon (C) is a major element that determines the strength of the material, if the carbon content is too low it is impossible to secure the required strength, if too high it is impossible to secure the required toughness and ductility, so the carbon content is 0.30-0.60 It is desirable to limit to%.

Silicon (Si) is added more than 0.10% to secure the necessary strength and solid deoxidation by solid solution strengthening, but if it is more than 0.40%, surface decarburization occurs in the material, so it is added at 0.10-0.40%.

Manganese (Mn) is added more than 0.8% to increase the strength of the steel due to the increase in hardenability and solid solution, but decreases the toughness when added more than 1.5%. Therefore, in order to obtain the required strength without significantly reducing the toughness, the content of the manganese is preferably limited to 0.8-1.5%.

Chromium (Cr) is an element necessary to ensure good quenching properties, in particular, to strengthen the matrix structure to increase the strength and also improve the wear resistance and corrosion resistance. In order to obtain such an effect, it is necessary to add 0.03% or more, but when it is added more than 1.0%, toughness is lowered and the price of the material is expensive, so it is preferable to limit the addition range to 0.03-1.0%.

Vanadium (V) combines with carbon and nitrogen in steel to form vanadium carbonitrides. The fine vanadium carbonitride produced during the cooling process is very effective in improving the strength. In order to obtain the required effect, it is necessary to add at least 0.03% or more, but if it is added more than 0.15%, the strength is increased more than necessary and the toughness and ductility are lowered. Therefore, it is preferable to limit the addition range to 0.03-0.15%.

Titanium (Ti) combines with nitrogen in steel to form titanium nitride. Titanium nitride improves the impact toughness of the steel by inhibiting austenite grain growth during reheating. If the amount of titanium added is too small, the absolute amount of titanium nitride is difficult to effectively inhibit grain growth, and if the amount exceeds a certain amount, the effect is saturated or rather reduced. Therefore, the appropriate addition amount is preferably in the range of 0.010-0.040%.

Aluminum (Al) is added for deoxidation and also to form aluminum nitride to refine the austenite to improve the impact toughness of the steel. If the added amount is small, the required effect is not obtained. If the added amount is large, the effect is saturated. Therefore, the added amount is preferably limited to the range of 0.01-0.10%.

Phosphorus (P) is segregated at the austenite grain boundary and degrades toughness, so the upper limit is preferably limited to 0.030%.

Sulfur (S) combines with manganese in the steel to form manganese sulfide. Manganese sulfide improves the free machinability of steel. In order to acquire such an effect, it is preferable to add 0.020% or more. However, since sulfur reduces impact toughness, it is preferable to limit the upper limit to 0.10%.

Nitrogen (N) combines with titanium, vanadium and aluminum in steel to form titanium nitride vanadium nitride and aluminum nitride, thereby improving strength and toughness. If the addition amount is small, such an effect cannot be obtained. If the addition amount is too large, the effect is saturated, so the addition range is preferably 0.005-0.030%.

Nickel (Ni) improves impact toughness by expanding the austenite region of steel when 0.15% or more is added to steel, but when the addition amount increases, low temperature metamorphic structure bainite or martensite occurs in the tissue, which adversely affects the cutting of products. It limits to the following.

[Manufacture process]

The steel of the composition as described above was cast in a conventional manner, and then heated to 1200-1300 ° C. in order to re-heat the coarse precipitated vanadium carbonitride at the time of reheating to prepare a billet. The billet is hot-rolled. The reheating temperature is preferably 9500-1100 ° C. The reason is that when the reheating temperature is higher than 950 ° C, the coarse precipitated vanadium nitride can be re-used, and when it exceeds 1100 ° C, austenite becomes coarse and decarburization becomes severe.

After reheating as described above, the hot wire is re-rolled and wound in a coil form, and then cooled. In this case, it is preferable to cool the 550-750 ° C section, which is a phase transformation section, at a cooling rate of 0.1-1.0 ° C / sec. If the cooling rate is slower than 0.1 DEG C / sec, the ferrite and pearlite structures become too coarse to reduce the strength and toughness, and the vanadium carbonitride precipitated during the cooling process is too grown to effectively contribute to the strength increase. And when the cooling rate is faster than 1.0 ℃ / sec is generated low temperature transformation tissue makes the steel vulnerable.

Wire rods manufactured in this way are cold drawn to fit the final product dimensions in order to generate compressive residual stresses on the wire surface.

After the cold drawing, the wire is cut to the appropriate length, and after the necessary cutting process, the surface is hardened by high frequency quenching and thinning treatment.

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

Example 1

Chemical compositions of the inventive steel and conventional steel applied to the embodiment of the present invention are shown in Table 2 below.

division Chemical composition (% by weight) Remarks C Si Mn P S Cr Al V Nb Ti N Ni Invention steel 0.45 0.25 1.20 0.015 0.050 0.09 0.032 0.10 - 0.016 0.0085 0.15 Conventional Steel 1 0.43 0.25 0.85 - 0.025 0.13 - 0.10 - - 0.01 - Japan Special Publication 59-136420 Conventional Steel 2 0.15 0.25 1.15 - - 0.12 0.02 - 0.11 0.015 - - Japanese Patent Laid Open Comparative steel 0.45 0.24 1.19 0.016 0.044 0.08 0.032 0.10 - 0.016 0.0105 -

Billets were prepared by charging cast slabs (250 × 330 mm) cast with steels as shown in Table 2 and heating them at a temperature of 1250 ° C. The billet was charged into a heating furnace, heated for 1 hour and 30 minutes, and extracted. At this time, the extraction temperature was 950 ° C. The extracted billet was rolled to a diameter of 30 mm using a hot wire rolling mill and then wound in a coil form. The wound wire rod was cooled in the 550-750 ° C section at a cooling rate of 0.25-0.10 ° C / sec using a cooling system. 1 shows a microstructure of the inventive steel which has completed hot wire rolling and adjusted cooling as described above.

The present invention steel shown in Fig. 1 (Fig. 1 (a) is 200 times, Fig. 1 (b) is 500 times), the low temperature transformation structure harmful to the material, such as bainite does not exist and is composed of fine ferrite and pearlite It shows microstructure.

In addition, the tensile strength and hardness results of wire diameter of the invention steel, conventional steel, etc. in the state of finishing hot wire rolling as described above are shown in Table 3 below.

division Tensile Strength (kg / mm ² ) Elongation (%) Hardness (HRc) Impact value (kg.m / cm ² ) Invention steel 89.1-97.0 11.0-17.0 23.4-25.3 6.5-7.5 Conventional Steel 1 - - 15.7-22.2 - Conventional Steel 2 69 - - - Comparative steel 86.0-96.0 12.0-18.0 21.4-24.3 5.5-7.0

As shown in Table 3, it can be seen that the inventive steel produced according to the present invention has improved mechanical properties compared to conventional steel and comparative steel.

As described above, the wire wound in the coil state was cold drawn in a cold state at a reduction rate of about 5-15%, and then cut to a predetermined length. After that, gear processing was performed on one side of the wire rod and the wire rod center was drilled in the longitudinal direction to manufacture a rack bar which is a vehicle steering device part. The surface of the shaft and the gear was subjected to high frequency quenching and heat treatment to improve wear resistance. The physical properties of the rack bar manufactured according to the present invention are shown together in Table 4 below as standard values of the properties of the wire rod (tempered steel wire) subjected to heat treatment.

division standard Invention steel Material straightness 0.40 / 1000mm or less 0.26 / 1000-0.30 / 1000mm Bending direction after broach processing Forward direction Forward direction Dental Hardness (HRc) 58-62 58.5-59.0 Shaft Hardness (HRc) 55-64 60-61

As shown in Table 4, it can be seen that the inventive steel has an equivalent physical property or higher than that of the existing tempered steel wire.

As described above, the present invention can provide a method for manufacturing high-frequency annealed nonferrous steel wire that is cost-saving and simplifies the manufacturing process, and the provided non-ferrous steel wire has a useful effect that can be applied to automobile rack bars and the like.

Claims (1)

By weight%, C: 0.30-0.60%, Si: 0.10-0.40%, Mn: 0.8-1.5%, Cr: 0.03-1.0%, V: 0.03-0.15%, Ti: 0.010-0.040%, Al: 0.01- Billet composed of 0.10%, P: 0.030% or less, S: 0.020-0.10%, N: 0.005-0.030%, Ni: 0.10-0.30% and the remaining Fe and other unavoidable impurities are heated to 950-1100 ° C. It is rolled in hot wire and wound in coil form, and then cools the 550-750 ° C section at a rate of 0.1-1.0 ° C / sec, and then cold draws at a section reduction rate of 5.0-30%. Method for manufacturing wire rod for shaft.