KR101254782B1

KR101254782B1 - Air hardening high strength machine structural steel without oil quenching and tempering after carburizing heat treatment and method producing the same

Info

Publication number: KR101254782B1
Application number: KR1020120035119A
Authority: KR
Inventors: 김동윤; 신정호; 류영주
Original assignee: 주식회사 세아베스틸
Priority date: 2012-04-04
Filing date: 2012-04-04
Publication date: 2013-04-15

Abstract

The present invention is an air hardening type mechanical structural part steel having a high strength omitting quenching and tempering, which is a heat treatment after carburization, and a manufacturing method thereof, wherein the weight% is C: 0.19∼0.25% by weight, Si: 0.90 to 1.10 wt%, Mn: 1.20 to 1.40 wt%, S: 0.010 to 0.025 wt%, Cr: 1.20 to 1.40 wt%, Mo: 0.07 to 0.13 wt%, V: 0.10 to 0.16 wt%, Al: 0.015 to 0.035% by weight, B: 0.0010% to 0.0030% by weight, Nb: 0.010% to 0.030% by weight, and Fe and inevitable impurities.
The present invention by the above configuration is conventional carburizing steel that has been used in the prior art to obtain the required strength through the oil quenching and tempering after carburizing heat treatment, the present invention is to increase the Si, add V, add Al, Nb This aims to improve the strength through the refinement of the particle size.

Description

Air hardening high strength machine structural steel without oil quenching and tempering after carburizing heat treatment and method producing the same}

The present invention relates to a low carbon-based air hardening type high strength mechanical structural component steel having a high strength and high toughness, and a method of manufacturing the same. More specifically, the tensile strength and the fatigue strength are reduced while the oil quenching and tempering are omitted. The present invention relates to a mechanical structural part steel 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, a low carbon alloy steel is usually hardened by carburizing heat treatment to produce parts having high wear resistance and internal toughness. In particular, due to the limited hardening of the surface during carburization heat treatment and the limitation of the material used, the interior of the surface other than the carburized portion shows the original ferrite + pearlite material, which is not excellent in fatigue strength.

As a comparative invention steel which omitted oil quenching after recently developed carburization heat treatment, European Patent No. 1098011, entitled "An air hardenable low to medium carbon steel composion", aims to reduce the carburizing time. As a main feature, the patent claims of carbon (C) were carried out at 0.10 to 0.55% by weight, but 0.39% by weight in order to achieve the object of the invention. In the range of 0.10 to 0.38% by weight, which is claimed by the comparative invention steel, the object of the invention of the comparative invention to reduce the carburizing time is considered impossible in consideration of the usual carburizing process.

In addition, the comparative invention steel uses expensive molybdenum (Mo) in order to secure strength during air hardening and omit quenching and tempering, and thus has a disadvantage in that the production cost is high.

It is an object of the present invention to provide a new low carbon air hardening type high strength mechanical structural component steel having high strength and high toughness, which can replace the low carbon alloy steel used for conventional automobile parts.

The object of the present invention is C: 0.19 to 0.25% by weight, Si: 0.90 to 1.10% by weight, Mn: 1.20 to 1.40% by weight, S: 0.010 to 0.025% by weight, Cr: 1.20 to 1.40% by weight, Mo: 0.07 ~ 0.13% by weight, Al: 0.015 ~ 0.035% by weight, V: 0.10 ~ 0.16% by weight, Nb: 0.010 ~ 0.030% by weight, B: 0.0010 ~ 0.0030% by weight and the rest are composed of Fe and unavoidable impurities, and after carburizing heat treatment It is achieved by a mechanical structural part steel characterized by omitting oil quenching and tempering.

In addition, the object of the present invention, C: 0.19 to 0.25% by weight, Si: 0.90 to 1.10% by weight, Mn: 1.20 to 1.40% by weight, S: 0.010 to 0.025% by weight, Cr: 1.20 to 1.40% by weight , Mo: 0.07 to 0.13% by weight, Al: 0.015 to 0.035% by weight, V: 0.10 to 0.16% by weight, Nb: 0.010 to 0.030% by weight, B: 0.0010 to 0.0030% by weight, and the rest are Fe and inevitable impurities And casting the steel having the composition, rolling the steel, hot working the rolled steel, and carburizing heat treatment of the hot worked steel. It is achieved by a method for manufacturing machined parts steel, characterized by eliminating quenching and tempering.

Preferably, the hot worked steel is subjected to a second process before the carburization heat treatment.

Preferably, the hot working is hot forging, hot forging in the temperature range of 1200 ℃ to 1250 ℃.

Preferably, after hot forging, cooling is performed at a temperature range of 800 ° C. to 850 ° C. at a cooling rate of 1 ° C./s.

Preferably, the second processing is less than 850 ℃ above 800 ℃ of cooling, after the cooling or after the cooling Ar ₁ It is carried out at temperatures below the transformation point.

As described above, the present invention has shortened the manufacturing process by omitting oil quenching and tempering after carburizing, compared to the existing steel, tensile strength and gear tooth fatigue strength can be improved compared to conventional steel to produce parts of high strength To make it possible.

1 is a view showing a gear, a shaft for an automobile manufactured by the invention steel.
2 is a graph showing the tensile strength of the invention steel and the comparative steel.
3 is a graph showing the hardenability of the inventive steel and the comparative steel.
4 is a diagram showing a tester and a test schematic diagram in which the gear tooth fatigue test of the inventive steel and the comparative steel is carried out.
5 is a graph showing the gear tooth fatigue strength of the inventive steel and the comparative steel.

The present invention relates to a low carbon-based air hardening type high strength mechanical structural component steel having a high strength and high toughness, and a method for manufacturing the same. In the comparative invention steel (European Patent No. 1098011), it is expensive to secure strength during air hardening. Molybdenum (Mo) is added 0.36 ~ 0.58% by weight, but in the present invention is added only molybdenum (Mo) 0.07 ~ 0.13% by weight, it is possible to reduce the cost. In the comparative invention, molybdenum is added in order to consider quenching and tempering. However, in the present invention steel, V, Al, Nb, and B were added after carburizing heat treatment instead of expensive Mo.

Hereinafter, the components added in the present invention will be described in detail.

C: 0.19 wt% to 0.25 wt%

C is one of the main elements determining strength and hardness in special steels, and it is necessary to contain C at least 0.19% by weight in order to secure strength. In addition, 0.19% by weight should be added to increase the diffusion rate of C during the carburization heat treatment. However, when it exceeds 0.25 weight%, forging workability and machinability will fall. Therefore, in consideration of these characteristics, the C content range is set to 0.19 to 0.25% by weight.

Si: 0.90 wt% ~ 1.10 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 0.90% by weight, the fatigue strength is insufficient. Lowers. Therefore, the content of Si is set to 0.90 to 1.10% by weight.

Mn: 1.20 wt% ~ 1.40 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 1.20 to 1.40% by weight.

S: 0.010 wt% ~ 0.025 wt%

S combines with Mn to form MnS to improve workability. However, when added in excess, it binds with FeS to embrittle the steel. Therefore, the content of S is set to 0.010 to 0.025% by weight.

Mo: 0.07 wt% ~ 0.13 wt%

Mo is an element that promotes bainite structure formation upon cooling after hot forging and is an important element for improving the hardenability of steel. If the content is less than 0.07% by weight, the hardenability is hardly achieved, and when added in an amount of 0.13% or more by weight, 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 0.07 ~ 0.13% by weight.

Cr: 1.20 wt% ~ 1.40 wt%

Cr is an element that increases the hardenability and makes carbides to increase impact resistance. The lower limit of Cr is 1.20% by weight to compensate for the hardenability by reducing Mn content and to increase the temper resistance by forming complex compounds with Mo, V and the like. When added over 1.40% by weight, the hardness is increased and workability is lowered. Therefore, the content of Cr is set to 1.20 to 1.40% by weight.

V: 0.10% to 0.16% by weight

V refines grains by fine carbonitride formation to improve strength and toughness. If the added amount is 0.10% by weight or less, the effect of increasing strength is small. If it is added more than 0.16% by weight, the strength is increased, but toughness is not only lowered, but it is not preferable because there is no economic effect due to the increase in manufacturing cost. Therefore, the V content is limited to 0.10% by weight to 0.16% by weight.

B: 0.0010% by weight to 0.0030% by weight

B is an element used to promote the formation of bainite structure, and according to Pickering et al., The addition of B significantly delays the formation of cornerstone ferrite in the time-temperature transformation diagram (TTT), thereby shifting the C curve of the cornerstone ferrite to the right. It has been reported. 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 0.0030% by weight.

Al: 0.015 wt% ~ 0.035 wt%

Al acts as a strong deoxidizer and combines with N to make grains finer, but less than 0.015% by weight of deoxidation or grain refinement is not preferred, 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 0.015 to 0.035% by weight.

Nb: 0.010 wt% ~ 0.030 wt%

Nb forms Nb (C) N with a strong affinity with C, N. Al added to refine the austenite grain size maintains the grain size up to 920 ° C, but at higher temperatures, the pinning effect of AlN is significantly lowered, which lowers the austenite grain size and adds Nb to the austenite grain size. . The content of Nb is set to 0.010 to 0.030% by weight.

[ Example ]

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 1 to 3 represent chemical compositions for steel grades manufactured using 100 kg of VIM (Vacuum Induction Melting) in various alloy designs set for development. Inventive Steel 4 shows the chemical composition of the mass-produced products designed to the optimum conditions through the evaluation of the VIM materials 1 to 3 produced by the alloy design. Comparative steels represent the components of steels currently in mass production.

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) A second process is carried out at 800 ° C. or higher and below 850 ° C. during cooling, or after cooling, or at a temperature below the Ar ₁ 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 heating of the carburizing heat treatment later. The second machining may correspond to rolling, forging, rolling, or the like.

d) The material is heated in a gas carburizing furnace to 900 ° C to 950 ° C to carburize the surface. After the carburizing heat treatment, the carbon concentration is 1 wt% on the surface, and is immediately cooled without quenching in oil. Omit tempering after air cooling.

* Each component content is expressed as wt%.

After heating and maintaining the invention steel and the comparative steel according to Table 1 at 920 ° C., the carburizing heat treatment temperature, the invention steel is cooled in the air, and the comparative steel is subjected to oil quenching and tempering, which is a common manufacturing process, and then Tensile specimens were processed and subjected to a tensile test using a tensile tester.

Figure 2 shows the results for this compared to the comparative steel subjected to oil quenching and tempering, the invention steel has a higher tensile strength than the comparative steel even though the oil quenching heat treatment was not performed.

Figure 3 is a result of the roughness test carried out to evaluate the hardenability of the steel, the comparative steel has a high hardness value of the surface through the water-cooled heat treatment, but from 9mm, which is a distance that exhibits the characteristics of ordinary automotive materials by water cooling While the quenching effect is reduced, the hardness value is lower than that of the inventive steel. In contrast, the invention steel exhibits a high hardness value while maintaining a constant distance from the surface to 50 mm, thereby exhibiting high performance when the steel is partized.

5 is a fatigue test results using the fatigue tester of Figure 4 evaluated the actual steel and the invention steel and comparative steel produced. Compared to the comparative steel which performed the oil quenching and tempering after the carburizing heat treatment, the inventive steel omitted the oil quenching and tempering after the carburizing heat treatment. Invented steel has higher gear tooth fatigue strength than comparative steel.

While the present invention has been particularly shown and described with reference to the particular embodiments thereof, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims

C: 0.20 to 0.24 wt%, Si: 0.90 to 1.10 wt%, Mn: 1.20 to 1.40 wt%, S: 0.010 to 0.025 wt%, Cr: 1.20 to 1.40 wt%, Mo: 0.07 to 0.13 wt%, Al: 0.015 to 0.035% by weight, V: 0.10 to 0.16% by weight, Nb: 0.010 to 0.030% by weight, B: 0.0010 to 0.0030% by weight, and the rest is composed of Fe and inevitable impurities,
Carburized heat-treated mechanical structural parts manufactured by cooling in air without oil quenching and tempering after carburizing heat treatment.

C: 0.20 to 0.24 wt%, Si: 0.90 to 1.10 wt%, Mn: 1.20 to 1.40 wt%, S: 0.010 to 0.025 wt%, Cr: 1.20 to 1.40 wt%, Mo: 0.07 to 0.13 wt%, Al: Casting a steel comprising 0.015 to 0.035% by weight, V: 0.10 to 0.16% by weight, Nb: 0.010 to 0.030% by weight, B: 0.0010 to 0.0030% by weight, and the balance of Fe and inevitable impurities;
Rolling the steel;
Hot working the rolled steel; And
Second machining the hot worked steel; And
Carburizing heat treatment of the second milled steel;
The second processing is rolling, forging or rolling, and is carried out at a temperature below 800 ° C. during cooling, at a temperature below Ar ₁ transformation point after cooling, or after cooling,
After the carburizing heat treatment, the carburizing heat treatment is characterized in that the cooling of the air to omit the oil quenching and tempering, characterized in that the manufacturing method for mechanical structural part steel.

3. The method of claim 2, wherein the hot working is hot forging and hot forging in a temperature range of 1200 ° C to 1250 ° C.

4. The method of claim 3, wherein after the hot forging, cooling is carried out at a temperature range of 800 ° C to 850 ° C at a cooling rate of 1 ° C / s.

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