KR20150072907A - Micro alloyed steel for a mechanical structure with high fatigue strength and excellent fracture splitting - Google Patents

Abstract

The present invention relates to microalloyed steel for a mechanical structure with high fatigue strength and excellent fracture splitting and, more specifically, to a high strength microalloyed steel achieving light weight of a connecting rod by improving fatigue strength, by using heat-treated steel and microalloyed steel to replace the existing connecting rod, and a manufacturing method thereof. The microalloyed steel comprises: 0.34 to 0.38 wt% of C; 0.90 to 1.10 wt% of Si; 0.90 to 1.10 wt% of Mn; 0.035 wt% or less of P; 0.06 to 0.10 wt% of S; 0.25 wt% or less of Cu; 0.20 wt% or less of Ni; 0.25 wt% or less of Cr; 0.06 wt% or less of Mo; 0.030 wt% or less of Al; 0.25 to 0.32 wt% of V; 0.055 to 0.090 wt% of Ti; 0.011 to 0.02 wt% of N; and the remainder consisting of Fe and inevitable impurities, wherein fatigue strength is 50 kgf/mm^2 or higher, yield strength is 800 MPa or higher, and tensile strength is 1000 MPa or higher.

Description

TECHNICAL FIELD [0001] The present invention relates to a mechanical structure non-tempered steel having high fatigue strength and excellent fracture splitting,

TECHNICAL FIELD The present invention relates to a non-tempered steel for mechanical structure and a method of manufacturing the same, and relates to a non-welded steel for a connecting rod having improved fatigue strength and light weight and a method for manufacturing the same.

In order to overcome global warming, which has become a global problem in recent years, regulation of CO ₂ gas emission and improvement of automobile fuel efficiency have been a constant concern. To this end, efforts are being made to reduce the weight of automobile parts, and to strengthen and improve their performance. The connecting rod, which is one of the automobile engine parts, is connected to the piston and reciprocates in the cylinder. It receives high temperature and high pressure gas pressure during the expansion process, and changes the reciprocating motion of the piston to rotational motion of the crankshaft.

The connecting rod is composed of two parts, namely, a cap and a rod. The connecting rod is mounted on the crankshaft by fastening them with bolts. In the past, the rod and the cap have been separated from each other by forging. However, tolerances for insertion of the assembly bolts, The gap between the rod and the cap shaft does not coincide with each other due to the gap between the rod and the cap, and the tight coupling becomes difficult. Therefore, when the crankshaft rotates at a high speed due to reciprocation of the piston during engine operation, the balance of weight is not achieved, which causes a problem of vibration, which generally results in poor operability of the engine.

In order to solve the above-mentioned problem, in recent years, a hot working method called 'fracture splitting method' in which a connecting rod is manufactured by rod and cap integral forging, Separable shorting type In comparison with the rod and cap, it is possible to solve the problems such as the manufacturing cost and the reduction in cost as well as the reduction in the cost due to the lack of seating and side surface processing. However, since C70S6 (Germany), which was developed as a conventional fracture-splitting connecting rod, has a fatigue strength of 35 kgf / mm ² and has a low durability and low cutting ability due to high carbon steel, Development is needed.

Korean Patent Laid-Open Publication No. 2001-0034363 discloses a steel sheet comprising 0.20 to 0.40 wt% of C, 0.10 to 1.50 wt% of Si, 0.30 to 1.40 wt% of Mn, 0.20 to 0.50 wt% of V, 0.150 wt% 50 to 200 ppm, the balance being iron and impurity-containing steel, and a process for producing the same, wherein the steel has a yield stress of 700 MPa or more and a tensile strength of 1100 MPa or less and a Re / Rm ratio of 0.73 or more, 7 J / cm ² or less Of the impact energy.

Korean Patent Laid-Open Publication No. 2009-0049642 discloses a steel sheet comprising 0.30 to 0.40 wt% of C, 0.50 to 0.80 wt% of Si, 0.90 to 1.20 wt% of Mn, 0.045 wt% or less of P, 0.06 to 0.10 wt% of S, A material having a composition of 0.30 wt% or less, Mo: 0.10 wt% or less, Ni: 0.20 wt% or less, Al: 0.040 wt% or less, V: 0.10-0.30 wt%, and N: 0.05-0.3 wt% / mm < ^{2 &} gt ;.

It is an object of the present invention to provide a non-tempered steel for a high strength mechanical structure having improved fatigue strength and excellent fracture toughness, and a method for manufacturing the same.

In order to solve the above-mentioned problems, the present invention provides a method of manufacturing a semiconductor device, comprising: 0.34 to 0.38 wt% of C, 0.90 to 1.10 wt% of Si, 0.90 to 1.10 wt% of Mn, 0.035 wt% 0.10 wt% Cu, 0.25 wt% or less Cu, 0.20 wt% or less of Ni, 0.20 wt% or less of Cr, 0.06 wt% or less of Mo, And a balance of Fe and inevitable impurities, wherein the steel contains 0.030 wt% or less (0 is not included), V is 0.25 to 0.32 wt%, Ti is 0.055 to 0.090 wt%, and N is 0.011 to 0.02 wt% do.

Preferably, the non-tempered steel for mechanical structure may have a fatigue strength of 50 kgf / mm ² or more, a yield strength of 800 MPa or more, and a tensile strength of 1000 MPa or more.

In addition,

0.34 to 0.38 wt% of C, 0.90 to 1.10 wt% of Si, 0.90 to 1.10 wt% of Mn, 0.035 wt% or less of P (not including 0), 0.06 to 0.10 wt% of S, 0.25 wt% or less of Cu (Not included), Ni: not more than 0.20 wt% (not included), Cr: not more than 0.20 wt% (not included), Mo: not more than 0.06 wt% A steel material containing 0.25 to 0.32% by weight of Ti, 0.055 to 0.090% by weight of Ti and 0.011 to 0.02% by weight of N and the balance of Fe and unavoidable impurities is melted and made into a molten metal. Subsequently, deoxidation and desulfurization, Producing a cast material through a continuous casting process; And

Reheating the cast material to a temperature of 1100 to 1300 占 폚 and forming the rolled material through a rolling process;

And a method for producing a non-treated steel for machine structural use.

The non-tempered steel for mechanical structure of the present invention is excellent in fatigue strength and has an effect of reducing weight by increasing strength. Further, the fracture splitting characteristic is excellent, and the damage of the connecting rod is prevented.

Fig. 1 is a graph showing the result of evaluation of rolling bending fatigue under the conditions of the invention steel and the comparative steel.
Fig. 2 is a photograph of a fracture section of a connecting rod manufactured by the mass production line after fracture of the invention steel A; Fig.
Fig. 3 is a photograph showing the microstructure of the inventive steel A by the position of the connecting rod manufactured in the mass production line.

Unless defined otherwise, all technical terms used in the present invention have the following definitions and are consistent with the meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Also, preferred methods or samples are described in this specification, but similar or equivalent ones are also included in the scope of the present invention. The contents of all publications referred to herein are incorporated herein by reference. The term " drug " is used in reference to a reference amount, level, value, number, frequency, percentage, dimension, size, amount, weight, or length of 30, 25, 20, 25, 10, 9, 8, 7, 6, Level, value, number, frequency, percentage, dimension, size, quantity, weight or length of a sample,

Throughout this specification, the words " comprising " and " comprising ", unless the context clearly requires otherwise, include the steps or components, or groups of steps or elements, And that they are not excluded.

The non-tempered steel for machine structural use according to the present invention contains 0.34 to 0.38 wt% of C, 0.90 to 1.10 wt% of Si, 0.90 to 1.10 wt% of Mn, 0.035 wt% or less of P, 0.06 to 0.10 wt% of S, Ti: 0.055-0.090 wt.%, N: 0.001 wt.% Or less, Ni: 0.20 wt.% Or less, Cr: 0.20 wt.% Or less, Mo: 0.06 wt.% Or less, 0.011 to 0.02% by weight, the balance being Fe and inevitable impurities.

Preferably, the non-tempered steel for mechanical structure may be a non-tempered steel for connecting rod.

Hereinafter, the components of the non-tempered steel for mechanical structure of the present invention will be described in detail.

C: 0.34 to 0.38 wt%

C is one of the main elements for determining strength and hardness in special steel, and it is necessary to add 0.34% by weight or more in order to secure strength. If the content of C exceeds 0.38% by weight, the toughness is lowered and the hardness is increased, resulting in poor cutting performance. If it is less than 0.34% by weight, it is difficult to secure the strength, so that it is necessary to add another alloy and the economical efficiency is insufficient. Therefore, the content of C is preferably 0.34 to 0.38% by weight.

Si  : 0.90 to 1.10 wt%

Si is used as an effective deoxidizer for steelmaking and it improves the fatigue strength by being employed in the base. Especially, ferrite strengthening element prevents fracture of cracks during fracture and induces fracture in grain boundary, and exhibits excellent fracture splitting property. If the content of Si exceeds 1.10% by weight, the possibility of brittleness of parts is generated. If the content of Si is less than 0.90% by weight, the ferrite strengthening effect may be deteriorated. Therefore, the content of Si is preferably 0.90 to 1.10% by weight.

Mn  : 0.90 to 1.10 wt%

Mn improves the denseness and strength of steel, and MnS is formed by bonding with S, thereby preventing red-hot brittleness and improving cutting workability. If the content of Mn is less than 0.90 wt%, it is difficult to obtain the above effect. When the content of Mn exceeds 1.10 wt%, a bainite structure is formed and the cutting performance is deteriorated. Therefore, the content of Mn is preferably 0.90 to 1.10% by weight.

P: not more than 0.035% by weight (not including 0)

P segregates at the grain boundaries of the austenite grain to lower the toughness and decrease the impact resistance, thereby increasing the fracture splitting characteristic. However, P is also an element that causes cracks in the process and lowers the strength. Therefore, the content of P is preferably 0.035% by weight or less.

S: 0.06 to 0.10 wt%

S combines with Mn in the steel to form MnS. MnS is formed to improve the machinability. However, an excessive amount of S added deteriorates fatigue strength and toughness. Therefore, the content of S is preferably 0.06 to 0.10% by weight.

Cu  : Not more than 0.25% by weight (not including 0)

Cu slightly increases the strength and hardness, but lowers the elongation. Particularly, when Cu is contained in an amount of 0.5% or more, it causes hot brittleness in hot working. Therefore, the content of Cu is preferably 0.25 wt% or less.

Ni  : Not more than 0.20% by weight (not including 0)

Although Ni improves the texture of the steel and significantly improves low-temperature toughness, it is preferable that the content of Ni is 0.20% by weight or less in order to increase the manufacturing cost of the steel.

Cr  : Not more than 0.25% by weight (not including 0)

Cr is an element that increases the incombustibility and improves the strength, and the excess toughness is lowered and the machinability is lowered. Therefore, the content of Cr is preferably 0.25% by weight or less.

Mo  : 0.06% by weight or less (not including 0)

Mo is very excellent in improving the incombustibility, and has an effect of improving the strength and toughness, but increases the manufacturing cost of the steel. Therefore, the content of Mo is preferably 0.06% by weight or less.

Al  : 0.030 wt% or less (not including 0)

Al acts both as a strong deoxidizer and at the same time it binds with N and serves to refine the crystal grains. If added in excess, the amount of non-metallic inclusions such as Al2O3 may increase, which may have detrimental effects such as lowering toughness. Therefore, the content of Al is preferably 0.030 wt% or less.

V: 0.25 to 0.32 wt%

V is an expensive element that combines with C and N to improve the fatigue characteristics of steel by precipitation strengthening effect by fine carbonitride. If it exceeds 0.32% by weight, the brittleness is increased and the production cost is increased. V improves the strength and toughness by precipitation strengthening effect by the addition of Ti and the composite. Therefore, the content of V is preferably 0.25 to 0.32% by weight.

Ti  : 0.055 to 0.090 wt%

Ti, like V, bonds with C or N to improve the fatigue characteristics of the steel by precipitation strengthening effect by fine carbonitride, and the machinability is deteriorated in excess. Ti improves strength and toughness by precipitation strengthening effect by addition of V and complex addition. Therefore, the content of Ti is preferably 0.055 to 0.090% by weight.

N: 0.011 to 0.02 wt%

N is combined with alloying elements such as V, Ti, and Al in the non-nitrided steel to form a nitride, thereby contributing to improvement of strength and toughness due to refinement of austenite grains. If it is less than 0.011% by weight, it may be difficult to form fine precipitates of V and Ti. If it is more than 0.02% by weight, the effect is saturated, and therefore the N content is preferably 0.011 to 0.02% by weight.

According to one embodiment of the present invention, there is provided a method for producing a non-tempered steel for mechanical structure, comprising the steps of: 0.34 to 0.38 wt% of C, 0.90 to 1.10 wt% of Si, 0.90 to 1.10 wt% of Mn, ), S: 0.06 to 0.10 wt%, Cu: not more than 0.25 wt% (not included), Ni: not more than 0.20 wt% (not included), Cr: not more than 0.20 wt% 0), Al: not more than 0.030% by weight (inclusive), V: 0.25 to 0.32% by weight, Ti: 0.055 to 0.090% by weight, N: 0.011 to 0.02% by weight and the balance Fe and inevitable impurities Melting the molten metal to form a molten metal, then subjecting the molten metal to deoxidation, desulfurization and vacuum degassing to produce a casting material through a continuous casting process; And

And reheating the cast material to a temperature of 1100 to 1300 캜 to produce a rolled material through a rolling process.

Example

Hereinafter, the present invention will be described in detail with reference to specific examples, but the scope of the present invention is not limited by the examples.

Table 1 shows chemical compositions and mechanical properties of Examples 1 to 5. The remainder consists of Fe and unavoidable impurities.

division Major Chemical Components
(weight%) Mechanical property
(MPa) Fatigue test
(kgf / mm ² ) C Si Mn P S Cu Ni Cr Mo Al V Ti N surrender
burglar Seal
burglar Rotational bending Example 1 0.36 1.05 1.05 0.022 0.073 0.17 0.07 0.14 0.02 0.009 0.32 0.069 0.013 880 1120 57 Example 2 0.38 0.95 1.00 0.016 0.065 0.16 0.09 0.12 0.01 0.007 0.27 0.073 0.012 842 1061 53 Example 3 0.37 0.99 1.00 0.020 0.122 0.19 0.07 0.12 0.01 0.008 0.28 0.069 0.014 827 1055 49 Example 4 0.38 1.07 0.95 0.026 0.091 0.14 0.05 0.09 0.01 0.007 0.28 0.004 0.015 703 960 47 Example 5 0.42 1.10 0.95 0.015 0.082 0.17 0.06 0.16 0.02 0.006 0.30 0.060 0.012 910 1140 58

Examples 1 to 5 were dissolved in a vacuum induction melting furnace (VIM), reheated and forged to about 32 mm thick. The forged material was maintained at 1200 ° C for 40 minutes, then subjected to air cooling, and subjected to a tensile test and a fatigue test according to No. KS4.

Examples 1 to 5 satisfied both the yield strength of 800 MPa or more and the tensile strength of 1000 MPa or more. In Example 3, the S content was high, and the fatigue strength in the fatigue test was lowered due to the effect of excessive MnS. In Example 5, the carbon content is relatively high, so that the workability is poor as compared with Examples 1 to 4. In Example 4, the Ti-free composition was insufficient in the effect of precipitation strengthening by the combined addition of V and Ti, and the mechanical properties required in the present invention were not satisfied.

Based on the above results, massive re-evaluation was performed considering the precipitation strengthening effect by the addition of V and Ti. Table 2 shows the chemical composition of inventive and comparative steels having the composition of the present invention. The remainder consists of Fe and unavoidable impurities.

division Main chemical composition (% by weight) C Si Mn P S Cu Ni Cr Mo Al V Ti N foot
persons
River A 0.36 1.00 0.98 0.021 0.070 0.21 0.07 0.17 0.01 0.009 0.29 0.056 0.011 B 0.35 0.97 0.96 0.015 0.075 0.17 0.06 0.15 0.01 0.007 0.30 0.057 0.012 ratio
School
River C 0.70 0.20 0.52 0.012 0.067 0.13 0.05 0.13 0.02 0.005 0.04 0.002 0.013 D 0.36 0.70 1.00 0.010 0.066 0.16 0.06 0.12 0.01 0.006 0.26 0.003 0.012

Invention steels A, B, and comparative steels C and D were melted in an electric furnace, continuously cast, heated at a temperature of 1100 to 1300 ° C, and then rolled into a Φ36 mm round bar. The specimens were held at 1200 ℃ for 40 min. After air cooling, tensile tests, tensile compression fatigue specimens and rotary bending fatigue specimens were prepared according to KS4.

Table 3 shows the mechanical properties and fatigue test results of the inventive steel and the comparative steel.

Mechanical property Fatigue test (kgf / mm ² ) Yield strength
(MPa) The tensile strength
(MPa) Elongation
(%) Section reduction rate (%) Surface hardness
(HB) Shock
(J / cm ² ) Rotational bending
(kgf / mm ² ) Tensile compression
(kgf / mm ² ) Inventive Steel A 822 1087 19 39 330 5 54 57 Invention steel B 817 1060 18 38 335 6 53 55 Comparative River A 515 911 13 18 250 23 36 37 Comparative Steel B 689 951 18 37 290 45 45 46

Referring to Table 3, it can be seen that the inventive steel is improved in both yield strength and tensile strength compared to the comparative steel by precipitation strengthening by the addition of V and Ti. Since the connecting rod receives tensile stress and compressive stress at the same time, it shows that the fatigue strength against tensile compression is very important and is about 40% higher than that of the comparative steel C, and about 20% higher than that of D. In the re-evaluation of invention steel, the impact value is 5 to 6 J / cm ² , but toughness is improved by promoting the formation of ferrite in the ingot by addition of Ti at 10 J / cm ² or more at the rod portion of the connecting rod forgings.

Fig. 3 is a photograph showing the microstructure of the inventive steel A by the position of the connecting rod manufactured in the mass production line. Referring to FIG. 3, when the connecting rod is fractured, the fracture surface thereof is destroyed by ferrite strengthening by the addition of Si to prevent fracture of the crack during fracture and induces fracture along the grain boundaries, thereby exhibiting cleavage fracture.

The present invention has been described with reference to the preferred embodiments. 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 as defined by the appended claims. Therefore, the disclosed embodiments should be considered in an illustrative rather than a restrictive sense. The scope of the present invention is defined by the appended claims rather than by the foregoing description, and all differences within the scope of equivalents thereof should be construed as being included in the present invention.

Claims (3)

 0.34 to 0.38 wt% of C, 0.90 to 1.10 wt% of Si, 0.90 to 1.10 wt% of Mn, 0.035 wt% or less of P (not including 0), 0.06 to 0.10 wt% of S, 0.25 wt% or less of Cu (Not included), Ni: not more than 0.20 wt% (not included), Cr: not more than 0.20 wt% (not included), Mo: not more than 0.06 wt% 0.25 to 0.32 wt.%, Ti: 0.055 to 0.090 wt.%, And N: 0.011 to 0.02 wt.%, With the balance Fe and unavoidable impurities. The method according to claim 1,
The non-tempered steel for mechanical structure has a fatigue strength of 50 kgf / mm ² or more, a yield strength of 800 MPa or more, and a tensile strength of 1000 MPa or more. 0.34 to 0.38 wt% of C, 0.90 to 1.10 wt% of Si, 0.90 to 1.10 wt% of Mn, 0.035 wt% or less of P (not including 0), 0.06 to 0.10 wt% of S, 0.25 wt% or less of Cu (Not included), Ni: not more than 0.20 wt% (not included), Cr: not more than 0.20 wt% (not included), Mo: not more than 0.06 wt% A steel material containing 0.25 to 0.32% by weight of Ti, 0.055 to 0.090% by weight of Ti and 0.011 to 0.02% by weight of N and the balance of Fe and unavoidable impurities is melted and made into a molten metal. Subsequently, deoxidation and desulfurization, Producing a cast material through a continuous casting process; And
Reheating the cast material to a temperature of 1100 to 1300 占 폚 and forming the rolled material through a rolling process;
Wherein the method comprises the steps of:

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