KR101280547B1 - Steel for mechanical and structural parts having ultra fine grain size after induction hardening and method of manufacturing the same - Google Patents

Abstract

PURPOSE: Mechanical structure part steel capable of having ultrafine grains after high frequency heat treatment and a manufacturing method thereof are provided to improve torsion fatigue lifetime and electrodynamic fatigue compared with existing steel, thereby manufacturing high strength parts. CONSTITUTION: Mechanical structure part steel has a hardened layer which is formed by high frequency heat treatment on at least a part of the surface, and the hardened layer contains austenite. The austenite has 5 ㎛ or less of the average grain size. The high frequency heat treatment is performed at 900-920°C, and then tempered at 100-150°C. The mechanical structure part steel is composed of, in wt%: C: 0.50-0.55: Si: 0.30-0.50; Mn: 0.35-0.55; Mo: 0.20-0.50; Cr: 0.10-0.30; V: 0.015-0.035; Al: 0.025-0.045; B: 0.0010-0.0050; Ti: 0.010- 0.050; and the remainder Fe and inevitable impurities. [Reference numerals] (AA) Example A; (BB) Example B; (CC) Example C; (DD) Comparative example

Description

Steel for Mechanical and Structural Parts having Ultra Fine Grain Size after Induction Hardening and Method of Manufacturing the Same

The present invention relates to a mechanical structural part steel having ultrafine grains and a method of manufacturing the same.

In general, automotive parts require high strength and high toughness due to the characteristics of the parts, and for this reason, parts are usually hardened in the middle carbon alloy steel through high frequency heat treatment to have high abrasion resistance, and internally tough parts. Particularly, due to the high heating temperature during high frequency heat treatment and the limitation of the material used, the austenite grain size of the surface usually shows a grain size of 15 to 30 μm, which is not excellent in fatigue strength. Recently, a method of obtaining a fine grain size by rapid heating by adjusting heating temperature and time, which is a variable of high frequency heat treatment, has a limitation in obtaining a fine grain size, JP 2004-8252, JP 204-210476, JP 2006- The comparative invention disclosed in 519625 also introduces cold processing to the structure of fine bainite or martensite before the high frequency heat treatment to improve the torsional strength. In high frequency heat treatment, the heating temperature is lowered and the heating time is shortened. Although steels with a thickness of µm were invented, this is a limited method.

JP 2004-8252, JP 204-210476, JP 2006-519625

SUMMARY OF THE INVENTION An object of the present invention is to provide a new medium carbon-based ultrafine steel having a high strength and high toughness and a method for manufacturing the same, which can replace the medium carbon-based alloy steel or carbon steel used for existing automobile parts.

The above object is achieved by a mechanical structural part steel having a hardened layer formed by high frequency heat treatment on at least a part of the surface, the hardened layer containing austenite, and having an average austenite particle diameter of the austenite of 5 µm or less. .

Another purpose as described above is C: 0.50 to 0.55% by weight, Si: 0.30 to 0.50% by weight, Mn: 0.35 to 0.55% by weight, Mo: 0.20 to 0.50% by weight, Cr: 0.10 to 0.30% by weight, V: 0.015 to 0.035% by weight, Al: 0.025 to 0.045% by weight, B: 0.0010 to 0.0050% by weight, Ti: 0.010% to 0.050% by weight, and the rest are composed of Fe and unavoidable impurities, while satisfying the above components At least one of the formulas (1) to (3) is satisfied, and the sum of C, Si, and Mo is achieved by the mechanical structural part steel characterized by containing 0.89% by weight or more.

C> 0.52% by weight ----- (1)

V> 0.020% by weight ---- (2)

Mo> 0.35 wt% ---- (3)

Another purpose as described above is C: 0.50 to 0.55% by weight, Si: 0.30 to 0.50% by weight, Mn: 0.20 to 0.50% by weight, Mo: 0.20 to 0.50% by weight, Cr: 0.10 to 0.30% by weight, V: 0.015 to 0.035% by weight, Al: 0.025 to 0.045% by weight, B: 0.0010 to 0.0050% by weight, Ti: 0.010% to 0.050% by weight, and the remainder is continuously cast a material having a composition of Fe and unavoidable impurities. Performing rolling to a size of 1, hot forging at a temperature of 1200 ℃ to 1250 ℃, cooling to a temperature range of 800 to 850 ℃ at a cooling rate of 1 ℃ / s after hot forging, 800 ℃ or more during cooling Performing a second process at a temperature below Ar1 transformation point after cooling, or after cooling, or after cooling, and considering at a temperature of 100 ° C to 150 ° C after performing a high frequency heat treatment at a temperature of 900 ° C to 920 ° C. It is achieved by a method of manufacturing a structural steel part including a.

Another purpose as described above is C: 0.50 to 0.55% by weight, Si: 0.30 to 0.50% by weight, Mn: 0.35 to 0.55% by weight, Mo: 0.20 to 0.50% by weight, Cr: 0.10 to 0.30% by weight, V : 0.015 to 0.035% by weight, Al: 0.025 to 0.045% by weight, B: 0.0010 to 0.0050% by weight, Ti: 0.010% to 0.050% by weight, and the remainder is continuously cast a material composed of Fe and inevitable impurities Rolling to a predetermined size, hot forging at a temperature of 1200 to 1250 ° C., quenching at 830 to 860 ° C. and boiling at 630 to 660 ° C., and at a temperature of 900 to 920 ° C. After the high frequency heat treatment is achieved by a method for producing a mechanical structural part steel comprising the step of considering at a temperature of 100 ℃ to 150 ℃.

As described in detail above, the steel of the present invention increases the C content in comparison with the existing steel, increases the surface strength through high-frequency heat treatment, and adds high Mo, V, and Ti to the high-temperature heat treatment using the precipitates and carbides of 5 μm or less. The torsional fatigue life was improved by obtaining ultra fine grain of, and the electric fatigue was also improved compared to the existing steel to manufacture high strength parts.

1 is a micrograph showing the grain size after high frequency heat treatment according to the alloying composition of the invention steel.
Figure 2 is a micrograph and TEM data showing the particle size of the invention steel according to Mo content.
3 is a schematic view showing a conventional steel and the invention steel manufacturing process.
4 is a micrograph showing the structure and grain size of the conventional steel and the invention steel.
5 is a graph showing the torsion fatigue strength of the conventional steel and the invention steel.
6 is a graph showing the electric fatigue strength of the conventional steel and the invention steel.
Figure 7 is a micrograph showing the grain size before the high-frequency heat treatment and the high-frequency heat treatment of the conventional steel and the invention steel.

Hereinafter, the setting of the alloy component of the present invention having the above content range and the reason for limitation of the component range will be described.

C: 0.50 wt% ~ 0.55 wt%

C is one of the main elements determining strength and hardness in special steels, and it is necessary to contain C at least 0.50% by weight in order to secure strength. In addition, in order to secure a high frequency heat treatment surface hardness of more than 700HV 0.50% by weight should be added. However, when it exceeds 0.55 weight%, forging workability and machinability will fall. Therefore, in consideration of these characteristics, the C content range is set to 0.50 to 0.55% by weight.

Si: 0.30 wt% ~ 0.50 wt%

Si is an element that is dissolved in the base and increases the fatigue strength through grain boundary strengthening. If the content is lower than 0.30% by weight, the fatigue strength is insufficient. If the content is higher than 0.50% by weight, Si promotes and embrittles the ferrite formation. Lowers. Therefore, the content of Si is set to 0.30 to 0.50% by weight.

Mn: 0.35 wt%-0.55 wt%

Mn improves yield strength by making pearlite fine and solidifying ferrite. It also improves the hardenability and strength of the steel, and increases the plasticity at high temperatures to improve castability. In particular, MnS is formed by combining with S, which is a harmful component, to prevent red brittleness and improve cutting processability. Therefore, the content of Mn is set to 0.35 to 0.55% by weight.

Mo: 0.20 wt% ~ 0.50 wt%

Mo is an element that promotes the formation of bainite structure upon cooling after hot forging and is an important element that refines the particle size during high frequency heat treatment by Mo carbide. If the content is 0.20% by weight or less, the amount of Mo precipitates is insufficient, so that the fineness of particles is not easily achieved, and when 0.50% by weight or more is added, the hardness is improved to reduce workability. In addition, Mo is an expensive element and it is necessary to appropriately adjust the content of addition. Therefore, the content of Mo is set to 0.20 to 0.50% by weight.

Cr: 0.10 wt% ~ 0.30 wt%

Cr is an element that increases the hardenability and makes carbides to increase the impact resistance. To lower the hardenability by reducing Mn content and to increase the temper resistance by forming complex compounds with Mo, V, the lower limit is 0.10% by weight. Addition of 0.30% by weight or more raises the hardness and decreases the workability. Therefore, the content of Cr is set to 0.10 to 0.30% by weight.

V: 0.015% to 0.035% by weight

V refines grains by fine carbonitride formation to improve strength and toughness. If the added amount is 0.015% by weight or less, the effect of increasing strength is small, and when it is added more than 0.035% by weight, the strength is increased, but toughness is lowered, and since it is not economical effect due to the increase in manufacturing cost, it is not preferable. Therefore, the V content is limited to 0.015% by weight to 0.035% by weight.

B: 0.0010% by weight to 0.0050% by weight

B is an element used to promote the formation of bainite structure. According to Pickering, B is significantly delayed in the formation of cornerstone ferrite in the time-temperature transformation diagram (TTT) when B is added. It has been reported to move to the right. This is known because B added to steel segregates at the austenite grain boundary as an atomic state, thereby lowering the grain boundary free energy, thereby suppressing the formation of the cornerstone ferrite. However, B has high affinity with nitrogen and oxygen to form oxides and nitrides when dissolving, and when heat-treating steels of such composition, borocarbide such as M ₂₃ (CB) ₆ and Fe ₂ B is formed It does not contribute to the inhibition of ferrite formation. Therefore, in the present invention, the content of B is set to 0.0010 to 0.0050% by weight.

Al: 0.025 wt% ~ 0.045 wt%

Al acts as a strong deoxidizer and combines with N to refine the grains, but at 0.025% by weight or less, deoxidation or grain refining becomes less desirable, and when excessively added, rather than nonmetallic inclusions such as Al ₂ O ₃ The increase can have a rather detrimental effect. Therefore, the appropriate content range of Al is limited to 0.025 ~ 0.045% by weight.

Ti: 0.010 wt% ~ 0.050 wt%

Ti forms TiN with strong affinity with N during steelmaking. When B added for bainite formation combines with N to form borocarb, the Ti is added to N to solve the problem because it does not contribute to suppressing the formation of the cornerstone ferrite. The content of Ti is set to 0.010 to 0.050% by weight.

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

Table 1 shows the chemical components of the inventive steel and the comparative steel having the composition of the present invention. Inventive steels A and B represent chemical compositions for steel grades manufactured using VIM (Vacuum Induction Melting) in various alloy designs set for development. Inventive steel C shows the chemical composition of mass-produced products designed under optimum conditions through the evaluation of VIM materials A and B manufactured in the alloy design.

In the following description, the manufacturing process is first melted in a 100 ton electric furnace, followed by refining and vacuum degassing, followed by a continuous casting process (heated at 1200 ° C to 1250 ° C, and reheated at 1200 ° C to 1250 ° C). Was prepared.

a) Hot-forging this round bar steel at 1200 degreeC-1250 degreeC.

b) After hot forging, cool to a temperature range of 800 ~ 850 ℃ at a cooling rate of 1 ℃ / s.

c) The second processing is carried out at 800 ° C. or higher and below 850 ° C. during cooling, or after cooling, or at a temperature below the Ar1 transformation point after cooling. When the second processing is performed here, the bainite structure and martensite structure can be refined, and the austenite particles can be further refined during the heating of the high frequency heat treatment later. The second machining may correspond to rolling, forging, rolling, or the like.

d) This material is heated to a high frequency heat treatment temperature of 900 ° C to 950 ° C at a heating rate of 100 ° C / s or more to perform high frequency heat treatment. When the heating temperature during the high frequency heat treatment is less than 900 ° C, the formation of austenite particles is insufficient, and a hardened layer is not formed. When the heating temperature is 950 ° C or higher, the growth of the austenite particles is not only promoted. The deviation of the austenite particle diameter becomes large, resulting in a drop in fatigue strength.

 e) The heat treatment for 90 minutes was performed at 150 degreeC by the heating furnace.

Each component content is expressed in wt%.

Fig. 1 shows the data obtained by evaluating the particle size after the high frequency heat treatment using the inventive steel and the comparative steel according to Table 1, respectively. Invention steels A and B using VIM and inventive steel C using mass production electric furnaces have a grain size of 5 µm or less, compared to 14 to 25 µm of grain size of comparative steel, thereby achieving the object of the invention.

FIG. 2 is an analysis of the mechanism of ultrafine grains using TEM for grain size showing significant difference after high frequency heat treatment of Inventive Steel C and Comparative Steel. In Comparative Steel, Invented Steel did not observe Mo precipitate which acts as grain boundary pinning. C shows the ultrafine grain of 5 micrometers or less by grain boundary peening by Mo precipitate.

3 is a diagram illustrating a process of obtaining ultrafine grain size through high frequency heat treatment using a developed steel C for a shaft manufactured using a high frequency heat treatment process using a conventional comparative steel.

Figure 4 is a photograph showing the structure and grain size according to the process step of the conventional comparative steel and the inventive steel.

Conventional comparative steel obtains fine grain size after high frequency heat treatment through cold working before high frequency heat treatment, which shows that the grains grow as shown in FIG. 4 when the bainite structure is not homogeneous during cold working. In order to solve this problem, a separate heat treatment should be involved. However, invented steel does not require cold forming after hot forging, and if the strength needs to be improved, it can be hardened at 830 ~ 860 ℃ and then soaked at 630 ~ 660 ℃. Also, through such heat treatment, grain size of 5 µm or less can be obtained.

5 shows the torsion fatigue strength of the conventional comparative steel and the inventive steel. As shown in FIG. 5, the fatigue strength of the conventional comparative steel is 50 KN, but the invention steel without quenching and soaking is about 60 KN and the invention steel after quenching and soaking is about 70 KN.

6 is a comparative evaluation of the comparative steel and the invention steel C used as a hub bearing by the electric fatigue test. In the Weibull distribution, which has a 10% probability of failure compared to conventional steel, invention steel C has a life span of 8.14 million, which is superior to 1.02 million of comparative steel. 7 shows the grain size before the high frequency heat treatment and the structure before the high frequency heat treatment of the electric fatigue specimen. As can be seen in Figure 7, the invention steel D shows a finer grain size before and after the high frequency heat treatment as compared with the conventional comparative steel B.

The mechanical structural part steel having the ultrafine grain according to the present invention can be used as an automotive part, and the manufacturing method of the mechanical structural part steel can be used to manufacture an automotive part.

B: bainite
TM: Temper Martensite
M: Martensite

Claims (2)

Machine structural parts steel
At least part of the surface has a cured layer formed by high frequency heat treatment, the cured layer contains austenite, and the austenite has an average austenite particle diameter of 5 µm or less,
The high frequency heat treatment includes a step of considering at a temperature of 100 ~ 150 ℃ after performing a high frequency heat treatment at a temperature of 900 ~ 920 ℃,
The part steel
C: 0.50 to 0.55% by weight, Si: 0.30 to 0.50% by weight, Mn: 0.35 to 0.55% by weight, Mo: 0.20 to 0.50% by weight, Cr: 0.10 to 0.30% by weight, V: 0.015 to 0.035% by weight, Al: 0.025 to 0.045% by weight, B: 0.0010 to 0.0050% by weight, Ti: 0.010% to 0.050% by weight and the rest are composed of Fe and unavoidable impurities, and satisfying the above components, the following formulas (1) to (3) Satisfy at least one of the formulas, and the sum of C, Si, and Mo contains 0.89% by weight or more,
Mechanical structural component steel, characterized by a torsional fatigue strength of 70 KN or more.
C> 0.52% by weight ----- (1)
V> 0.020% by weight ---- (2)
Mo> 0.35 wt% ---- (3) delete

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