KR100368552B1 - Hot forged steel with low material deviation and its manufacturing method - Google Patents

Abstract

The present invention relates to a method for manufacturing hot forged steel having a low material deviation, and more particularly, to a hot forged steel having a low material deviation, which is used as a material for a part manufactured through a hot forging process among parts of automobiles and industrial machinery. It relates to a manufacturing method. In the present invention, by weight%, C: 0.20 to 0.40%, Si: 0.10 to 0.60%, Mn: 0.70 to 2.0%, Cr: 0.25 to 1.0, Ni: 0.1 to 0.5, V: 0.05 to 0.20%, N: 0.0050 ~ 0.020, P: 0.030% or less, S: 0.030% or less, hot-rolled steel made of residual Fe and other unavoidable impurities in a rod form breaks the casting structure, and then reheats to 1100 to 1300 ° C, followed by hot Forging is carried out, and after hot forging, characterized in that the natural cooling or forced air cooling.

Description

Hot forged steel with low material deviation and its manufacturing method {omitted}

The present invention relates to a hot forging non-ferrous steel and a method of manufacturing the same, with a low material deviation, in particular, a hot forging non-ferrous steel used for the material of the parts produced by the hot forging process of the parts of automobiles and industrial machinery And to a method for producing the same.

In general, high-strength automotive and industrial machinery parts are reheated to a high temperature of about 1200 to 1250 ° C. as shown in FIG. 1, and hot forged to a final part shape in a heated state and then cooled. After that, a hardening and annealing treatment is performed to secure the required strength and toughness. The hardening and heat treatment during the above-described process is a quenching process in which a forged product is heated to about 850 ° C., followed by water cooling or oil cooling to harden the material, and a toughness is given to the hardened steel by heating and cooling it to about 600 ° C. again. It is divided into Tempering process. Conventional hot forging steel material processed through such a process is usually used as carbon steel for mechanical structures such as S45C, S50C as shown in Table 1 below, the microstructure in the final product state after the hardening and annealing treatment is tempered Martensite (Temperod martensite).

TABLE 1

However, since the mid 70's, forged steel for hot forging has been developed and used, which can drastically reduce cost by eliminating hardening and heat treatment mainly in Europe and Japan, and the demand is continuously increasing. The characteristics of the known non-ferrous steel for hot forging are described in detail below.

As a representative example, non-coarse steels developed in Japan are steels containing 0.40 to 0.55% of carbon and 0.10% of vanadium (V), a precipitate forming element. It has about 93kg / mm ² . If these steels are to be used as a substitute for existing heat-treated steel, special cooling equipment should be provided to ensure that the cooling rates of the individual forgings are uniform during the cooling of these steels after hot forging. If the cooling of the forged part is not constant thickness of each part without such a cooling facility, the mechanical properties may be different for each part, the mechanical properties may vary depending on the position in another part. This is because the mechanical properties are changed by changing the final microstructure and precipitation state when the cooling rate is changed during cooling after forging.

All the non-steels developed at home and abroad have such problems, so it is not difficult to use them even if there is a special cooling facility as mentioned above or any quality deviations by product or location. There was a limitation that only available.

The present invention is to solve the above-described problems, by appropriately adjusting the steel grade components to minimize the mechanical property changes according to the change in cooling rate to provide a car or industrial machine parts with little material deviation without installing a special uniform cooling equipment It is an object of the present invention to provide a non-coarse steel for hot forging and a method for producing the same, which can be produced with low material deviation.

1 is a manufacturing process diagram of a conventional hot forging component;

Figure 2 (a) and (b) is a microstructure photograph according to the cooling rate of the present invention steel.

In order to achieve the above object, according to the present invention, the forged steel for hot forging with low material deviation is in the weight%, C: 0.20 to 0.40%, Si: 0.10 to 0.60%, Mn: 0.70 to 2.0%, Cr: It is composed of 0.25 to 1.0, Ni: 0.1 to 0.5, V: 0.05 to 0.20%, N: 0.0050 to 0.020, P: 0.030% or less, S: 0.030% or less, balance Fe and other inevitable impurities.

In addition, according to the present invention, the method for producing hot forged steel having a low material deviation is to hot-roll the steel composed of the above components in the form of rods to break the casting structure, and then reheat it to 1100 to 1300 ° C. and then forge in hot. After the hot forging, characterized in that the natural cooling or forced air cooling.

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

The forging steel for hot forging according to the present invention is by weight, C: 0.20 to 0.40%, Si: 0.10 to 0.60%, Mn: 0.70 to 2.0%, Cr: 0.25 to 1.0, Ni: 0.1 to 0.5, V: 0.05 0.20%, N: 0.0050 to 0.020, P: 0.030% or less, S: 0.030% or less, balance Fe and other inevitable impurities.

Hereinafter, the reason for limitation of the component range of an additional element and hot forging conditions in this invention is demonstrated in detail.

Carbon (C) is an element necessary for increasing the strength of steel. 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 impact value, it is preferable to limit the carbon content to 0.20 to 0.40%.

Silicon (Si) is added up to 0.60% in order to increase the strength by solid solution strengthening, but if added above, the toughness is rather impaired. If the addition amount is small, it is preferable to add 0.10% or more because it is difficult to secure sufficient deoxidation and required strength.

Manganese (Mn) is added to increase the strength of the steel by increasing the hardenability and enhancing the solid solution. However, addition of more than 2.0% reduces toughness. Therefore, the content of manganese is preferably limited to 0.70 to 2.0% in order to obtain the required strength without significantly reducing the toughness.

Chromium (Cr) is added to increase the hardenability and increase the strength of the steel in the fast cold rate range. However, when added more than 1.0%, the hardenability is excessively increased to produce martensite rather than bainite, which may degrade the impact toughness of the steel excessively. If the amount is small, it is difficult to secure necessary quenchability, so it is preferable to add 0.25% or more.

Nickel (Ni) is added to increase the strength of the steel by increasing the hardenability and strengthening the solid solution. However, nickel is an expensive element and if it is added too much, the strength is excessively increased, so it is preferable to limit the upper limit to 0.5%. If the added amount is less than 0.1%, it is difficult to secure the required strength, so the content of nickel is preferably limited to 0.1 to 0.5%.

Vanadium (V) effectively increases the hardness of the steel by depositing fine nitrides and carbonitrides during the cooling process if the cooling rate is properly adjusted. In the case of adding a large amount, it is preferable to limit the addition range to 0.05 to 0.20% because the toughness is reduced, the price of steel is high, and the addition amount is small, so that the precipitation strengthening effect is insignificant.

Nitrogen (N) combines with vanadium and / or forms vanadium nitride and / or vanadium carbonitride to refine the structure of the steel to strengthen the steel. However, since the steel becomes brittle when a large amount is added, it is preferable to limit the addition range to 0.0050 to 0.020%.

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

Sulfur (S) is an element segregated well and lowers the toughness of the steel, so it is preferable to limit the addition amount to 0.030% or less.

Hot-rolled steel is rod-shaped to break the cast structure, and then reheated and forged in hot. After hot forging, natural cooling or forced air cooling.

Further, according to the present invention, the method for producing hot forged steel is cast and then reheated and hot rolled in the form of a rod. The larger the reduction ratio is, the better it is to destroy the casting structure in this process. The hot rolled material is reheated to 1100 ~ 1300 ℃ and forged with various types of parts in the heated state.

At this time, when the reheating temperature for performing the hot forging operation is higher than 1300 ℃, the austenite grains are excessively grown to reduce the toughness of the steel, while if lower than 1100 ℃, the forging temperature is lowered significantly reducing the die life. Let's do it. Therefore, the reheating temperature before forging is appropriately 1100 to 1300 ° C.

In addition, when the forging temperature during hot forging is lower in the austenite range, the impact toughness is improved, but when it is too low, the die life is severely shortened, so the range of 900 to 1100 ° C. is preferable.

After forging, forced air cooling in the form of natural air cooling in the atmosphere or forced air blowing. At fast cooling rates such as forced air cooling, the internal structure of the steel becomes a mixed structure of ferrite and pearlite containing bainite due to the influence of quenching hardening elements chromium and nickel. Strengthening is accomplished by knight organization. At a slow cooling rate such that the heavy forged product is naturally air cooled, fine vanadium carbonitride precipitates in the mixed structure of ferrite and pearlite, and in this case, the strength is increased by precipitation strengthening.

In summary, the reinforcing mechanism described so far shows that when the cooling rate is high, bainite, which is a low temperature hard tissue, is formed, and the strength is increased, and when the cooling rate is slowed, the bainite structure decreases while precipitation of vanadium carbonitride increases. The strength is increased. Thus, in a certain cooling rate range, the strength remains constant regardless of the cooling rate change.

The present invention will be described in more detail with reference to the following Examples.

EXAMPLE

Inventive steels and comparative steels, such as the components shown in Table 2 below, were made of small ingots (160 mm × 160 mm × L), and then heated at 1200 ° C. for 2 hours and then rolled hot to prepare 60 mm thick plates. Thereafter, the specimen was reheated to 1100 ° C. to simulate the forging process, and then cooled to room temperature at various cooling rates (1 ° C./min to 1000 ° C./min).

TABLE 2

The microstructures of the inventive steels and the comparative steels prepared as described above were observed and their mechanical properties were investigated.

Figure 2 (a) is a microstructure of the present invention steel is composed of coarse ferrite and pearlite when cooled at a cooling rate 10 ℃ / min, Figure 2 (b) is a fine ferrite when cooled at a cooling rate 100 ℃ / min It is a microstructure of the present invention in which bainite, which is a low temperature transformation structure, is mixed in the pearlite structure.

Table 3 below lists the mechanical properties of the inventive steel and the comparative steel for each cooling rate.

TABLE 3

In the case of the comparative steel, the hardness increases proportionally as the cooling rate increases. This is because the microstructure is sequentially changed to ferrite + pearlite-> bainite-> martensite as the cooling rate increases. In contrast, in the case of the present invention, although the hardness tends to increase as the cooling rate increases, there is a section showing an almost constant hardness value without increasing the hardness even when the cooling rate increases. That is, in the cooling rate of 10 ° C / min and 100 ° C / min section, the hardness is a constant value of about 280 ~ 290Hv. This shows the precipitation strengthening effect of fine precipitation of vanadium carbonitride in the ferrite + pearlite structure at a cooling rate of about 10 ° C / min, and hardly the vanadium carbonitride was hardly precipitated at 100 ° C / min. This is because the bainite structure, which is a structure, is partially formed in the ferrite + pearlite structure to strengthen the steel.

Therefore, in the case of using the inventive steel, even if the cooling rate is changed within the range of about 10 to 100 ° C / min after hot forging, homogeneous mechanical properties can be obtained.

According to the present invention, by appropriately adjusting the steel grade component to minimize the change in mechanical properties according to the change of cooling rate for hot forging that can produce automobile parts or industrial machinery parts with little material deviation without installing a special uniform cooling equipment It is effective to produce non-crude steel.

Claims (2)

By weight%, C: 0.20 to 0.40%, Si: 0.10 to 0.60%, Mn: 0.70 to 2.0%, Cr: 0.25 to 1.0%, Ni: 0.1 to 0.5%, V: 0.05 to 0.20%, N: 0.005 to 0.020%, P: 0.030% or less, S: 0.030% or less, balance Fe and other inevitable impurities A non-alloyed steel for hot forging with low material deviation, characterized in that it has a crystal structure in which vanadium carbonitride is precipitated in ferrite + pearlite structure or bainite structure is mixed. By weight%, C: 0.20 to 0.40%, Si: 0.10 to 0.60%, Mn: 0.70 to 2.0%, Cr: 0.25 to 1.0%, Ni: 0.1 to 0.5%, V: 0.05 to 0.20%, N: 0.005 to 0.020%, P: 0.030% or less, S: 0.030% or less, hot-rolling steel composed of residual Fe and other inevitable impurities to form a rod to break the cast structure; Performing hot forging in a reheated state at 1100 to 1300 ° C., After hot forging, the step of cooling at a cooling rate of 10 ~ 100 ℃ / min, A method for producing hot forged steel with low material deviation, characterized in that it has a crystal structure in which vanadium carbonitride is precipitated in ferrite + pearlite structure or bainite structure is mixed.