KR100470671B1 - A method for manufacturing non-hteat-treated steel with excellent cold formability - Google Patents

Abstract

The present invention relates to a method for manufacturing cold-rolled non-annealed steel, which is used as a material for automobile steering apparatus, and the like, by adjusting the steel component, rolling and cooling conditions to bainite the structure of the wire rod, thereby eliminating heat treatment. It is an object of the present invention to provide a method for manufacturing non-alloyed steel having excellent cold pressure composition.

The present invention for achieving the above object, in weight%, C: 0.15-0.35%, Si: 0.1% or less, Mn: 1.0-2.0%, P: 0.03% or less, S: 0.03% or less, Cr: 0.20 ~ 1.0%, Mo: 0.10 to 0.50%, Ni: 0.2 to 1.0%, V: 0.05 to 0.20%, Al: 0.02 to 0.10%, remainder Fe and other unavoidable impurities include 1000 to 1200 ° C After reheating the furnace, the method of producing non-coated steel with excellent cold-pressure composition comprising hot rolling with a wire rod at a finish rolling temperature of 900 to 1050 ° C. and cooling at a cooling rate of 0.1 to 10 ° C./sec. do.

Description

A method for manufacturing non-alloyed steel with excellent cold rolling process {A METHOD FOR MANUFACTURING NON-HTEAT-TREATED STEEL WITH EXCELLENT COLD FORMABILITY}

The present invention relates to a method for manufacturing cold-rolled non-annealed steel, which is used as a material for automobile steering apparatus, and more particularly, to greatly improve the cold-pressure composition of a material by changing the microstructure of the steel wire to bainite. It relates to a method for producing non-coarse steel.

In general, the parts of the automobile steering apparatus are made of medium carbon low alloy steel wire (SCM435) having the components as shown in Table 1, which has a large tensile strength immediately before cold rolling, and thus cold forging after the spheroidizing heat treatment. It was then subjected to quenching and heat treatment to produce the product.

Chemical composition (% by weight) C Si Mn P S Cr Mo 0.35 0.25 0.65 0.03 0.03 1.0 0.2

In the manufacture of conventional automotive parts as described above, since the tensile strength in the state of wire rod rolling is about 100 kg / mm ² , and the sectional reduction rate is very low, about 35%, the material is softened and the elongation is increased before cold rolling, thereby improving workability. A spheroidizing heat treatment was required to make it work. The spheroidizing heat treatment is to heat the medium carbon low alloy steel wire to about 740 ° C. for several hours, and then cool it to about 600 ° C. at a slow cooling rate to spheroidize the internal microstructure of the steel wire. It's about 15 to 20 hours. Therefore, there has been a problem in that the productivity is lowered and the manufacturing cost is increased.

In order to solve the problems as described above, techniques for producing a heat treatment omitted steel wire material has been proposed, for example, Japanese Patent Application Laid-Open No. 1-165721, by weight% C: 0.12 ~ 0.17%, Si: 0.15 Steels containing ~ 0.35%, Mn: 1.3 ~ 1.6%, Cr: 0.25% or less, and V: 0.08! 0.13% are heated to 1000 ~ 1050 ℃ and then 750 ~ 850 ℃ with reduction rate of 50% or more in finishing rolling stage. The present invention discloses a wire rod manufacturing method for spheroidizing heat treatment and quenching treatment treatment, characterized in that the wire rod is rolled and then cooled at a cooling rate of 4.5 to 6.5 ° C./sec. In this technology, since the steel wire material having excellent strength and toughness can be manufactured through rolling and cooling control, there is an advantage that the required mechanical properties in the final product state can be obtained without heat treatment. However, since the microstructure of the finished wire rolling is composed of ferrite and pearlite and contains Si, which lowers the cold press composition, at a level of ordinary materials, the pressure load during the cold press is large and die consumption in the cold press process is large. It seems to be. In addition, the implementation of this technique requires a powerful rolling mill capable of rolling steel wire at a low temperature of 750 ~ 850 ℃, and also has a problem of managing the cooling rate very narrowly to reduce the material deviation.

In another technique, Japanese Patent Laid-Open No. 9-67622 contains, by weight, C: 0.15 to 0.35%, Si: 0.05% or less, Mn: 0.70 to 1.50%, N: 0.005% or less, and Cr: 0.20% or less. Disclosed is a method for producing wire rod for omission of spheroidizing heat treatment and quenching treatment, characterized in that the steel is cooled at a cooling rate of 2 ° C / sec or more after hot rolling, and then 20 to 35% of freshness is added. In the case of this technique, the cold pressure composition was improved by excluding Si, which lowered the cold pressure composition. However, since the cooling rate after rolling is faster than 2 ° C / sec, it is manufactured by bar-in-coil. In large diameter wire rods (diameter of 14 mm or more), such a fast cooling rate cannot be obtained with a conventional installation, and thus it is difficult to apply.

Accordingly, the present inventors have repeatedly conducted research and experiments to solve the above problems and propose the present invention based on the results, and the present invention adjusts the steel component, rolling and cooling conditions to cut the structure of the wire rod. It is an object of the present invention to provide a method for producing non-coated steel, which can omit the heat treatment and has excellent cold pressure composition.

The present invention for achieving the above object, in weight%, C: 0.15-0.35%, Si: 0.1% or less, Mn: 1.0-2.0%, P: 0.03% or less, S: 0.03% or less, Cr: 0.20 ~ 1.0%, Mo: 0.10 to 0.50%, Ni: 0.2 to 1.0%, V: 0.05 to 0.20%, Al: 0.02 to 0.10%, remainder Fe and other unavoidable impurities include 1000 to 1200 ° C The present invention relates to a method for manufacturing non-coated steel with excellent cold-pressure composition, which comprises hot rolling with a wire rod after finishing reheating to 900 to 1050 ° C, and cooling at a cooling rate of 0.1 to 10 ° C / sec.

Hereinafter, the steel component and manufacturing conditions of the present invention will be described.

Carbon (C) of the present invention is an element having a great influence on the strength of the steel, and if the content is less than 0.15%, the target strength cannot be secured. If the carbon (C) is more than 0.35%, the toughness of the steel exceeds the target strength range. Since it reduces, it is preferable to set the content to 0.15 to 0.35%.

Silicon (Si) is usually added for deoxidation during steelmaking, and also secures the strength required for the product. However, when the Si content is 0.1% or more, it is not preferable because the deformation resistance during cold working greatly increases and the cold pressure composition is sharply dropped.

Manganese (Mn) is an element that increases the hardenability of steel and has a structure refining effect, and increases the strength of steel without deteriorating impact toughness. By adding the manganese, in order to increase the hardenability of the steel and to stably obtain bainite structure even in a slow cooling rate section, the content should be set to 1.0% or more. However, when the added amount is more than 2.0%, the toughness decreases, so it is preferable to limit the added range of manganese to 1.0 to 2.0%.

Phosphorus (P) is segregated at the grain boundary, and the toughness of the steel is reduced, it is preferable to limit the content to 0.03% or less.

Sulfur (S) reduces the impact toughness of the steel, it is desirable to limit the content to 0.03% or less.

Chromium (Cr) is an element that acts to stably obtain bainite structure by increasing the hardenability of steel, and when the content is small, it is difficult to obtain such an effect, and when the addition amount is large, brittle martensite structure is generated. The content of is preferably set to 0.2 ~ 1.0%.

Molybdenum (Mo) is an element that increases the hardenability of the steel to stably obtain the ultra low carbon bainite structure. If the content is small, it is difficult to obtain such an effect, and the addition amount increases the strength of the steel excessively. Preference is given to adding in%.

Nickel (Ni) increases the hardenability of the steel to obtain a bainite structure stably, and serves to increase the strength of the steel without reducing the toughness. When the content of nickel is low, such an effect is hardly obtained, and when the amount of addition is large, the strength of the steel is excessively increased. Therefore, the content is preferably set to 0.2 to 1.0%.

Vanadium (V) is an element that combines with carbon in the steel to form carbides to increase the strength of the steel. If the amount is small, it is difficult to obtain such an effect. If the amount is large, the steel is embrittled. Therefore, the vanadium (V) is preferably added at 0.05 to 0.20%. .

Aluminum (Al) is an element that deoxidizes in the steelmaking process. When the amount is small, it is difficult to obtain such an effect. When the amount is large, aluminum oxide (Al ₂ O ₃ ) is formed to cause nozzle clogging during casting. It is preferable to set the content range to 0.02 to 0.1%.

After reheating the steel strips prepared as described above at a temperature of 1000 to 1200 ° C., the wires are rolled in hot, and finish-rolled at a temperature range of 900 to 1050 ° C., and then cooled at a rate of 0.1 to 10 ° C./sec.

The reason why the reheating of the steel sheet is performed at a temperature range of 1000 to 1200 ° C. is that if the heating temperature is 1200 ° C. or more, the austenite grains become coarse, which not only lowers the toughness of the final product but also causes severe surface decarburization. This is because the surface hardness and durability are severely lowered. If the heating temperature is less than 1000 ° C., the hot deformation resistance of the material is rapidly increased, and thus the hot rolling cannot be efficiently performed, thereby reducing the productivity.

The reason why the finish rolling is carried out at a temperature range of 900 to 1050 ° C. is that when the finish rolling temperature is 1050 ° C. or more, the austenite structure is excessively grown before transformation, thereby increasing the probability of generating martensite, which is brittle. This is because the toughness of the river is bad. When the finish rolling temperature is less than 900 ° C, the rolled structure becomes too fine and the hardenability decreases, and a soft ferrite structure is generated, thereby rapidly decreasing the strength.

The hot rolled wire rod is cooled under the above conditions, and the cooling rate is preferably 0.1 to 10 ° C / sec. The reason is that if the cooling rate is slower than 0.1 ° C./sec, ferrite structure occurs during transformation and the strength of the steel is greatly reduced. This is because it excessively increases and decreases toughness.

The wire rod cooled as described above is composed of bainite structure and has excellent elongation, so that spheroidization heat treatment can be omitted while having a strength similar to that of the prior art before cold rolling. In other words, in terms of workability, it is preferable that the tensile strength of the wire rod state is low and the tensile strength of the wire rod state is high. In the related art, the tensile strength of the wire rod state is high and the cross-sectional reduction rate is low, thereby softening the tissue by performing spheroidization heat treatment. There was a need. However, according to the present invention, since the structure of the wire rod state is bainite structure, the strength is high, but the elongation is good, the workability is excellent, the spheroidization heat treatment is unnecessary, and the life of the die can be prevented from being shortened. Since such high strength extends to a product, there is also an advantage in terms of the product.

On the other hand, the wire, after the cold drawing is processed by a conventional method, the forging is processed into a product.

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

(Example)

After the steel composition as shown in Table 2 was produced into a small steel piece of 110 mm x 110 mm x 250 mm, it was heated at 1100 ℃ for 2 hours, hot-rolled to finish the finish rolling at 950 ℃. The comparative steel of Table 2 is a case where the spherical heat treatment is omitted from the conventional non-coated steel, which excludes Si and improves the cold pressure composition. Then, after cooling the wire at a cooling rate of 0.5 ℃ / sec, the microstructure and tensile strength of the wire was observed, the results are shown in Figures 1 (a), (b) and 2.

Thereafter, the inventive steel and the comparative steel wire rod were cold drawn without spherical heat treatment, and then cold forged to be processed into a product.

C Si P S Mn Cr Mo Ni Al V Ti Invention steel 0.26 0.05 0.009 0.007 1.48 0.51 0.20 0.49 0.037 0.14 - Comparative steel 0.47 0.04 0.007 0.005 1.80 - - - - 0.11 0.017

As shown in Figs. 1 (a) and (b), the inventive steel of Fig. 1 (a) has a needle-shaped bainite structure, while no bainite-promoting elements such as Cr, Mo, and Ni are contained. The comparative steel of 1 (b) shows that the main structure consists of pearlite. That is, in the case of a structure made of pearlite, since the cold press composition deteriorates later, it is not preferable.

Figure 2 shows the tensile strength and cross-sectional reduction rate of the invention steel and the comparative steel after hot rolling, Figure 2 also shows the tensile strength and cross-sectional reduction rate of the medium carbon low alloy steel (SCM435) subjected to conventional spheroidizing heat treatment. As shown in Figure 2, the conventional heat-treated steel medium carbon low alloy steel (SCM435) has a high tensile strength of 100kg / mm ² in the wire state, while the low cross-sectional reduction rate of about 35%, the spheroidized heat treatment is performed to reduce the cross-sectional reduction rate 50 Had to increase by more than%. The reason is that if the cross-sectional reduction rate is less than 50%, cold pressing is impossible due to the brittleness of the material. On the other hand, the inventive steels and comparative steels of the present invention had a tensile strength of 86.2kg / mm ² and 92.2kg / mm ² in the wire state, and the sectional reduction ratios exceeded 50% and 70%, respectively. It can be seen that. As a result of rolling the inventive steel and the comparative steel wire rod of the present invention at a cold reduction ratio of 14%, the tensile strength of the inventive steel increased to 97.5 kg / mm ² , and the comparative steel increased to 102.3 kg / mm ² . Since this is higher than 95 kg / mm ^{2 which} is a tensile strength value obtained after quenching the conventional medium carbon low alloy steel, the inventive steel and the comparative steel of the present invention can replace the medium carbon low alloy steel which is a conventional heat treatment steel.

However, it can be seen that the comparative steel has a higher deformation resistance than that of the inventive steel, so that the cold pressure composition is inferior. That is, in order to have excellent cold pressure composition, the deformation resistance should be small. As shown in FIG. 3 comparing the deformation resistance of the comparative steel and the invention steel, the comparison steel has a larger deformation resistance value than the invention steel.

On the other hand, Figure 4 is a graph comparing the cold-pressure composition through the superiority of the strain resistance values of the two steels purely, excluding the effect of the high and low tensile strength on the deformation resistance by the same tensile strength value. As shown in FIG. 4, it can be seen that the inventive steel has a low cold rolling characteristic value (= strain resistance / tensile strength) and thus has excellent cold rolling composition.

As described above, according to the present invention, it is possible to effectively manufacture automotive parts in cold without spheroidizing heat treatment before cold working, and also harden the heat treatment after the processing of the product, thereby reducing the heat treatment cost, shortening the delivery time and It is possible to obtain effects such as productivity improvement.

Figure 1 (a), (b) is a microstructure photograph of the invention steel and comparative steel in the hot rolled state

2 is a graph showing the tensile strength and the reduction of section of the inventive steel, the comparative steel, and the conventional steel in the hot rolled state;

Figure 3 is a graph showing the deformation resistance according to the strain of the invention steel, comparative steel

Figure 4 is a graph showing the cold pressure composition according to the strain of the invention steel and the comparative steel

Claims (2)

By weight%, C: 0.15 to 0.35%, Si: 0.1% or less, Mn: 1.0 to 2.0%, P: 0.03% or less, S: 0.03% or less, Cr: 0.20 to 1.0%, Mo: 0.10 to 0.50%, Ni: 0.2 ~ 1.0%, V: 0.05 ~ 0.20%, Al: 0.02 ~ 0.10%, remainder Fe and other unavoidable impurities are reheated to 1000 ~ 1200 ℃, then finish rolling temperature is 900 ~ A method for producing an amorphous steel having excellent cold rolling composition comprising hot rolling at a wire temperature of 1050 ° C and cooling at a cooling rate of 0.1 to 10 ° C / sec. The method of claim 1, wherein the structure of the wire rod after hot rolling is a bainite structure.