KR960006455B1 - Making method of high strength high toughness hot forging steel with no-thermal refining - Google Patents

Abstract

The steel, composed of in weight 0.2-0.5 % C, 0.4 -0.8 % Si, 1.0 -2.0 % Mn, 0.05 -0.5 % Cr, 0.01-0.1 % Mo, 0.02 -0.1% Ni, 0.05-0.2% V, 0.01-0.04% Ti, 0.01-0.1% Al, 0.005 -0.03% N, less than 0.03% P, 0.02-0.1 % S, balance Fe and other impurities, is treated to make high strength, high tensile strength hot forging steel without quenching and tempering ; after hot rolling to break forged cell structure, it starts forging at 1100- 1300 deg.C, finishs at 900 deg.C, cools down to 400 deg.C at rate of 10-200 deg.C/min. and air cooling.

Description

Manufacturing method of non-coated steel for high strength, high toughness hot forging

1 is a microstructure photograph of a steel produced by the method of the present invention.

The present invention relates to a method for producing a high strength, high toughness hot forging non-steel, and more particularly to a method for manufacturing a high strength, high toughness hot forging non-ferrous steel is omitted. High-strength, high-toughness hot forging is used for many parts of vehicles such as crankshafts, connecting rods, hubs, knuckle arms, and front axles.

Conventionally, such parts have re-heated the rolled material to a high temperature of about 1250 ° C., and then hot forged in a hot state, and then subjected to a hardening and annealing process, thereby securing necessary strength and toughness.

The hardening and soaking heat treatment during the process is a quenching process to harden the material by heating it to about 850 ° C., followed by water cooling or oil cooling, and to give toughness to the hardened steel by heating it to about 600 ° C. and cooling it again. ) Process.

Conventional hot forging steels processed through such processes are usually used in mechanical structural carbon steel such as S45C and S50C or mechanical structural alloy steel such as SCM435 as shown in Table 1 below. It has a microstructure of Tempered Martensite and has the mechanical properties shown in Table 2.

Table 1

[Table 2]

However, from the mid 1970s, hot forged non-coated steel, which can significantly reduce cost by eliminating quenching and annealing heat treatment in Europe and Japan, has been considered and has been mass-produced since the mid-1980s. For example, Japanese Patent Publications Nos. 55-138056 and 56-156717 describe hot forged non-steel for containing about 0.1% of vanadium (V) as a precipitation hardening element.

The reason for adding vanadium is to strengthen the steel by finely depositing vanadium carbonitride in the cooling process after forging. In addition, these non-alloyed steels contain about 1.5% of manganese (Mn) in large amounts in order to take advantage of the properties of manganese, which enhances the strength without reducing toughness. However, these steels have a similar strength to that of hardened and hardened mechanical structural carbon steels, but the impact toughness is much lower than that of heat treated steels. Therefore, these hot forged steels have been used only in a limited number of parts where impact toughness is hardly a problem.

The non-alloyed steel developed to overcome this problem is a non-toughened steel for high toughness hot forging as shown in SO 61-166919, SO 62-199750, SO 62-253725 and the like. These high toughness hot forging steels improve impact toughness by changing various components or changing hot working conditions. That is, in Small 61-166919 and Small 62-199750, the finish forging temperature was performed at a low temperature of 800 ° C. or lower to greatly improve impact toughness. However, if these methods are applied to the existing forging process that performs the finish forging at about 1100 ℃, there is a problem that the cost reduction effect by omitting the heat treatment is almost lost because the die wear is huge. In 62-253725, meanwhile, the manganese content is reduced to the normal level of medium carbon steel, and instead, a small amount of titanium (Ti) is added to improve toughness at high temperatures. However, this steel has a problem that the tensile strength (80kg / ㎜) is relatively low because excessively limit the amount of alloy elements such as manganese in order to prevent the deterioration of toughness. That is, it is difficult to use in parts requiring high strength and high toughness by dropping the strength too much to improve impact toughness, and there is a problem in that reliability is lowered in terms of strength when replacing existing heat-treated steel.

Accordingly, an object of the present invention is to provide a method of manufacturing an improved non-manufactured steel for hot forging that solves the conventional problems as described above.

In the present invention, to adjust the various components appropriately to produce a high-strength, high toughness hot forging non-steel for excellent strength and toughness without lowering the forging temperature.

According to the present invention, an amorphous steel for high strength and high toughness hot forging is prepared.

According to the present invention, in weight%, C: 0.20-0.50%, Si: 0.40-0.80%, Mn: 1.0-2.0%, Cr: 0.05-0.50%, Mo: 0.01-0.1%, Ni: 0.02-0.10%, V: 0.05-0.20%, Ti: 0.01-0.04%, Al: 0.01-0.10%, N: 0.005-0.030%, P: 0.030% or less, S: 0.02-0.10%, balance Fe and other unavoidable impurities Hot-rolled or bar-rolled steel to break the cast structure, and then reheated to 1100-1300 ℃ and then hot forged to finish forging at a temperature above 900 ℃, and then 10-200 to 400 ℃. Provided is a method for producing a high strength, high toughness hot forged non-coated steel, which is cooled at a cooling rate of ° C / min and then air cooled.

Hereinafter, the method of the present invention will be described in detail.

The carbon (C) is much lower than the conventional heat treatment materials (S45C, S50C, SCM435, SCM440) to increase the volume fraction of the ferrite to increase the impact toughness and ductility of the steel, but is required when the carbon content is too low Since the strength cannot be secured, the carbon content is preferably limited to 0.20-0.50%.

Silicon (Si) is added up to 0.80% in order to increase the strength due to solid solution strengthening and increase the toughness by increasing the fraction of ferrite and improving the distribution. In order to ensure the required strength and to sufficiently deoxidize, it is preferable to add 0.40% or more.

Manganese (Mn) increases the hardenability and lowers the transformation temperature from austenite to ferrite, thereby miniaturizing the tissue.It does not degrade the toughness even when added to about 2.0% in ordinary steel. It is preferable to limit the content of the manganese to 1.0-2.0% in order to increase the strength without increasing the toughness without increasing the toughness.

Chromium (Cr) increases the hardenability of the steel, increasing its strength. If it is less than 0.05%, the strengthening effect is not sufficient, and if the amount is large, it is difficult to secure the target toughness and economic efficiency, so the content of chromium is preferably limited to 0 05-0.5%.

Molybdenum (Mo) converts the river's structure into a needle-shaped ferrite to strengthen the river. If the addition amount is too small, the effect is not sufficient, and if the addition amount is large, it is not economical, so the content of molybdenum is preferably limited to 0.01-0.1%.

Nickel (Ni) is dissolved in steel to increase strength. If the added amount is too small, the effect is not sufficient, and if the added amount is large, there is no economical effect. Therefore, the nickel content is preferably limited to 0.02 to 0.1%.

Vanadium (V) is added to form fine carbonitrides in the cooling process after forging to improve strength. If the addition amount is too small, the strength increase range is small, and when the addition amount is large, the strength is increased, but toughness is greatly reduced, so it is preferable to limit the addition amount to 0.05-0.20%.

Titanium (Ti) combines with nitrogen in steel to form titanium nitride. Titanium nitride improves the impact toughness of steel by inhibiting austenite grain growth during reheating before forging.

If the amount of titanium added is too small, it is difficult to effectively inhibit grain growth due to the absolute amount of titanium nitride, and if the amount exceeds a certain amount, the effect is saturated. Therefore, it is not necessary to add an expensive alloy element excessively. Therefore, the titration amount is in the range of 0.010-0.040%.

Aluminum (Al) is added for the purpose of particle oxidation by deoxidation and aluminum nitride. When the addition amount is small, the effect is not sufficiently exhibited, and when the addition amount exceeds a certain amount, the effect is saturated, so it is preferable to limit the addition amount to 0.01-0.10%.

Nitrogen (N) combines with vanadium, titanium, and aluminum to form nitrides in the crude steel. If the added amount is 0.005% or less, sufficient nitride is not formed. If the added amount is 0.030% or more, the effect is saturated. Therefore, it is preferable to limit the added amount to 0.005-0.030%.

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. Since manganese sulfide increases the machinability of steel, it is preferable to add at least 0.02% of sulfur in order to improve the workability of non-coated steel. However, when the added amount is large, toughness is lowered, so it is preferable to limit the upper limit to 0.10%.

After casting the steel of the composition as described above, and then reheated, the steel sheet rolling or bar rolling.

Since this process means sizing rolling to an appropriate size for forging, the rolling temperature and the rolling reduction ratio are not a big problem. Since the casting structure is destroyed in this process, the heating temperature is higher and the reduction ratio is better. After the rolled or rolled steel, the material is reheated and hot forged. If the reheating temperature for hot forging is too high, the austenite particles will grow excessively, leading to a decrease in toughness, and if too low, the forging temperature will be lowered, thereby significantly reducing the die life. Therefore, the reheating temperature before forging is appropriately 1100-1300 ℃. Since the forging temperature during hot forging, the lower the in the austenite range, the better the toughness of impact, the lower the temperature can be lowered to the Ar ₃ temperature (about 900 ℃).

In the case of the cooling rate after the forging, if the quenching is more than 200 ℃ / min can form a low-temperature structure and harm the toughness. .

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

(Example)

The steel ingot (size 145mm × 145mm × L) as shown in Table 3 was hot-rolled to a total pressure reduction rate of 45% after 2 hours at 1200 ° C. to prepare 80 mm × 150 mmj × L steel pieces. The rolled steel pieces were reheated at 1250 ° C. and hot forged at a total pressure drop of 70.84%. Forging start temperature was set to 1200 ℃ and forging temperature was controlled to 1100 ℃. Final thickness of the forging material was 13mm, 25mm, it was air-cooled after forging. The air cooling rates of the forgings according to thickness were 42 ° C / min and 21 ° C / min, respectively (400-800 ° C section average cooling rates). For reference, the chemical composition of the comparative steel is also shown in Table 3 together with the chemical composition of the inventive steel.

Table 3

The microstructures of the specimens prepared as described above were observed and their mechanical properties were investigated, respectively.

Table 4

The microstructure photograph of FIG. 1 shows the microstructures of the inventive steel 1 and the hardened steel 2 when cooled at a cooling rate of 42 ° C./min. According to this, in the case of the present invention, the structure is composed of a complex structure of ferrite and pearlite.

In addition, the invention steel does not contain low temperature structure which adversely affects the mechanical properties despite the addition of a large amount of manganese and other alloying elements.

In addition, according to Table 4, it can be seen that the steel of the present invention has excellent strength and toughness despite the high forging temperature of 1100 ° C.

As described above, according to the method of the present invention, it is possible to produce a high-strength, high-toughness hot forging non-ferrous steel excellent in rigidity and toughness without undergoing a hardening and annealing process.

Claims (1)

By weight%, C: 0.20-0.50%, Si: 0.40-0.80%, Mn: 1.0-2.0%, Cr: 0.05-0.50%, Mo: 0.01-0.1%, Ni: 0.02-0.10%, V: 0.05- Steel composed of 0.20%, Ti: 0.01-0.04%, Al: 0.01-0.10%, N: 0.005-0.030%, P: 0.030% or less, S: 0.02-0.10%, balance Fe and other unavoidable impurities Hot-rolled or bar-rolled to destroy the cast structure, and then reheated to 1100-1300 ℃ and hot forged to finish forging at a temperature of 900 ℃ or higher, and then 10-200 ℃ / min up to 400 ℃. A method for producing a high strength, high toughness hot forged steel, characterized in that the cooling at a cooling rate and then air cooled.