KR20150076886A

KR20150076886A - Carburizing steel and method of manufacturing the same

Info

Publication number: KR20150076886A
Application number: KR1020130165565A
Authority: KR
Inventors: 이상원; 남궁승; 박철우
Original assignee: 현대제철 주식회사
Priority date: 2013-12-27
Filing date: 2013-12-27
Publication date: 2015-07-07

Abstract

Disclosed are a carburizing steel capable of reducing a time for carburizing heat treatment by facilitating a generation of carbide with 1.0 wt% or less of carbon by adding 3.00-5.50 wt% of chrome (Cr) of alloying constituents; and a manufacturing method thereof. According to the present invention, the method to manufacture the carburizing steel comprises: (a) a step of preparing a forging material consisting of 0.10-0.30 wt% of carbon (C), 0.05-0.8 wt% of silicon (Si), 0.4-1.5 wt% of manganese (Mn), 0.015 wt% or less of phosphorous (P), 0.010 wt% or less of sulfur (S), 3.00-5.50 wt% of chrome (Cr), 0.005-0.040 wt% of aluminum (Al), 0.005-0.040 wt%, of niobium (Nb), 0.001 wt% or less of oxygen (O), 0.015 wt% or less of nitrogen (N), and the remainder consisting of iron (Fe) and inevitable impurities; (b) a step of performing carburizing heat treatment on the forging material at a temperature of 900-950°C; (c) a step of oil-quenching the carburizing heat-treated forging material; and (d) a step of tempering the oil-quenched forging material.

Description

TECHNICAL FIELD [0001] The present invention relates to a carburizing steel,

The present invention relates to a carburizing steel and a method of manufacturing the same, and more particularly, to a method of manufacturing a carburizing steel and a method of manufacturing the same by increasing the content of chromium (Cr) in the alloy component to 3.00-5.50 wt% To a carburizing steel which can shorten the heat treatment time for carburizing and a manufacturing method thereof.

The carburizing heat treatment process generally refers to a method of increasing the surface hardness of the steel to thereby improve the abrasion resistance of a part where friction is generated.

Recently, a high concentration carburizing method has been developed for miniaturization and high performance of parts. The high-concentration carburizing method refers to a method of producing carbide to be sprayed on a martensite base by containing 2 to 3% by weight, which is about 2-3 times higher than that of a carburizing steel containing 0.8 to 1.0% by weight of carbon .

However, such a high concentration carburizing method requires a carburization time of 10 hours or more due to an increase in carbon content, which is a cause of high cost and time.

Techniques related to the present invention include carburizing steel, carburizing steel parts and a manufacturing method thereof disclosed in Korean Patent Laid-Open Publication No. 10-2013-0037228 (published on April 13, 2013).

It is an object of the present invention to provide a method for producing a carbide having an excellent abrasion resistance and fatigue strength by shortening a carburizing heat treatment time by adding a chromium (Cr) content of 3.00-5.50 wt% And to provide a method for producing the same.

In order to achieve the above object, the present invention provides a method of manufacturing a carburized steel, comprising: (a) 0.10 to 0.30% carbon, 0.05 to 0.8% silicon, manganese (Mn) (P): 0.015% or less, S: 0.010% or less, Cr: 3.00 to 5.50%, Al: 0.005 to 0.040%, Niobium: Providing a forging material comprising 0.040% oxygen (O): 0.001% or less, nitrogen (N): 0.015% or less, and remaining iron (Fe) and unavoidable impurities; (b) carburizing the forging material at 900 to 950 캜 for 1 to 2 hours; (c) oiling the carburized heat treated forging material; And (d) tempering the oil quenched forging material.

In order to achieve the above object, the carburized steel according to an embodiment of the present invention may contain 0.10 to 0.30% carbon, 0.05 to 0.8% silicon, 0.4 to 1.5% manganese (Mn) (P): not more than 0.015%, sulfur (S): not more than 0.010%, chromium (Cr): 3.00 to 5.50%, aluminum (Al): 0.005 to 0.040%, niobium (Nb) O): not more than 0.001%, nitrogen (N): not more than 0.015%, and the balance of iron (Fe) and unavoidable impurities, and has a surface hardness of 60 HRC or more and a core hardness of 490 Hv or more.

The carburized steel according to the present invention and the method for manufacturing the same can reduce the time for carburizing heat treatment by promoting the generation of carbide by adding carbon content of 1.0 wt% or less by adding chromium (Cr) content of 3.00-5.50 wt% have.

Therefore, the carburized steel produced by the method according to the present invention has a surface hardness of 60 HRC or more and a core hardness of 490 Hv or more, and has excellent wear resistance and fatigue strength.

FIG. 1 is a flowchart showing a method of manufacturing carburized steel according to an embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention and the manner of achieving them will become apparent with reference to the embodiments described in detail below with reference to the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, To fully disclose the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout the specification.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a carburizing steel according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

The carburized steel according to the present invention contains 0.10 to 0.30% of carbon, 0.05 to 0.8% of silicon (Si), 0.4 to 1.5% of manganese (Mn), 0.015% of phosphorus (P) (S): not more than 0.010%, chromium (Cr): 3.00 to 5.50%, aluminum (Al): 0.005 to 0.040%, niobium (Nb): 0.005 to 0.040%, oxygen (O) N): 0.015% or less and the balance of iron (Fe) and unavoidable impurities.

The carburizing steel may further include at least one of molybdenum (Mo): 0.01 to 0.4%, boron (B): 0.001 to 0.004%, and titanium (Ti): 0.010 to 0.040%.

At this time, the carburized steel has a surface hardness of 60 HRC or more and a deep hardness of 490 Hv or more.

Hereinafter, the role and content of each component contained in the carburized steel according to the present invention will be described.

Carbon (C)

Carbon (C) contributes to securing strength and contributes to improvement of surface hardness by carburizing heat treatment.

The carbon (C) is preferably added in a content ratio of 0.10 to 0.30% by weight of the total weight of the carburized steel. If the addition amount of carbon (C) is less than 0.10 wt%, it is insufficient to secure hardness by the strength and carburization. On the other hand, when the addition amount of carbon (C) exceeds 0.30% by weight, the toughness of the center part is lowered due to the increase in the hardness of the center part.

Silicon (Si)

Silicon (Si) is an element that inhibits the settling property. However, silicon is essentially included because it acts as a deoxidizer.

The silicon (Si) is preferably added in a content ratio of 0.05 to 0.8% by weight of the total weight of the carburized steel. When the addition amount of silicon (Si) is less than 0.05% by weight, the effect of deoxidation is insufficient. On the other hand, when the addition amount of silicon (Si) exceeds 0.8% by weight, the sticking property may be largely lowered.

Manganese (Mn)

Manganese (Mn) contributes to the improvement of steel strength.

The manganese (Mn) is preferably added at a content ratio of 0.4 to 1.5 wt% of the total weight of the carburized steel. When the addition amount of manganese (Mn) is less than 0.4% by weight, the effect of addition thereof is insufficient. On the other hand, when the addition amount of manganese (Mn) exceeds 1.5% by weight, grain boundary oxidation may occur and the mechanical properties of the carburized steel may be lowered.

In (P)

Phosphorus (P) contributes to strength improvement but worsens weldability and can cause final material deviation by center segregation. Therefore, in the present invention, the content of phosphorus (P) is limited to 0.015% by weight or less based on the total weight of the carburized steel.

Sulfur (S)

Sulfur combines with manganese to form MnS, which contributes to improvement in machinability.

The sulfur (S) is preferably added in an amount of 0.010 wt% or less based on the total weight of the carburized steel. If the addition amount of sulfur (S) exceeds 0.010 wt%, the fatigue strength of the steel is deteriorated.

Chromium (Cr)

Chromium (Cr) contributes to improving the ingotability of the steel.

The chromium (Cr) is preferably added in a content ratio of 3.00 to 5.50 wt% of the total weight of the carburized steel. Particularly, in the present invention, the content of chromium (Cr) is 1.5 times or more as compared with the conventional method, so that the generation of carbide is accelerated only by the carbon content at a low concentration after the carburization, thereby shortening the carburizing heat treatment time.

If the added amount of chromium (Cr) is less than 3.00 wt%, it may be difficult to exhibit the promoting effect of carbide production. On the other hand, when the addition amount of chromium (Cr) exceeds 5.50 wt%, the toughness is lowered and the occurrence of grain boundary oxidation is facilitated.

Aluminum (Al)

Aluminum (Al) combines with nitrogen to form AlN and contribute to refinement of the austenite grains.

The aluminum (Al) is preferably added at a content ratio of 0.005 to 0.040% by weight of the total weight of the carburized steel. When the addition amount of aluminum (Al) is less than 0.005% by weight, the effect of addition is insufficient. On the contrary, when the addition amount of aluminum (Al) exceeds 0.040 wt%, the fatigue strength of the steel can be inhibited.

Niobium (Nb)

Niobium (Nb) contributes greatly to grain refinement and to increase the temperature at which recrystallization is stopped, thereby enabling high-temperature carburization.

The niobium (Nb) is preferably added in an amount of 0.005 to 0.040% by weight of the total weight of the carburized steel. When the addition amount of niobium (Nb) is less than 0.005% by weight, the effect of the addition is insufficient and it is difficult to carry out the high temperature carburizing heat treatment. On the other hand, when the addition amount of niobium (Nb) exceeds 0.040 wt%, the toughness of the steel can be lowered without further effect.

Oxygen (O)

Oxygen (O) is an unavoidable impurity in the present invention, and when it is contained in an amount exceeding 0.0010% by weight, deterioration of the cleanliness of the steel is deteriorated to cause deterioration of contact fatigue. Therefore, in the present invention, the content of oxygen (O) is limited to 0.0010 wt% or less of the total weight of the carburized steel.

Nitrogen (N)

Nitrogen (N) combines with aluminum, niobium and the like and contributes to miniaturization of austenite grains.

However, when the content of nitrogen (N) exceeds 0.015 wt%, the physical properties of the steel can be inhibited. Therefore, nitrogen (N) is limited to 0.015 wt% or less of the total weight of the carburized steel according to the present invention.

Molybdenum (Mo)

Molybdenum (Mo) contributes to improving the strength and toughness of the steel.

The molybdenum (Mo) is preferably added in a content ratio of 0.1 to 0.4% by weight based on the total weight of the carburized steel. If the addition amount of molybdenum (Mo) is less than 0.1% by weight, the effect of the addition is insufficient. On the other hand, when the addition amount of molybdenum (Mo) exceeds 0.4% by weight, it is possible to inhibit the machinability of the steel and the toughness of the center portion.

Boron (B)

Boron (B) is an element for improving hardenability.

The boron (B) is preferably added at a content ratio of 0.001 to 0.004% by weight of the total weight of the carburized steel. When the content of boron (B) is less than 0.001% by weight, the effect of addition may be insufficient. On the contrary, when the content of boron (B) exceeds 0.004% by weight, the toughness and ductility of steel are deteriorated.

Titanium (Ti)

Titanium (Ti) has the effect of improving the toughness and strength of steel by reducing the austenite grain growth by welding Ti (C, N) precipitates with high stability at high temperatures, thereby finishing the welded structure.

The titanium (Ti) is preferably added in an amount of 0.010 to 0.040 wt% of the total weight of the carburized steel. When the content of titanium (Ti) is less than 0.010% by weight, there arises a problem that aging hardening occurs due to the remaining solid carbon and nitrogen employed without precipitation. On the contrary, when the content of titanium (Ti) exceeds 0.040% by weight, coarse precipitates are produced, which lowers the low-temperature impact properties of the steel and raises manufacturing costs without further effect of addition.

Manufacturing method of carburizing steel

Referring to FIG. 1, a method of manufacturing carburized steel according to an embodiment of the present invention includes a forging material preparation step S110, a carburizing heat treatment step S120, an oil quenching step S130, and a tempering step S140 .

Forging

In the forging material preparation step (S110), a forging material having the above composition is provided.

At this time, the forging material may be a steel manufactured through processes such as steelmaking, performance, and rolling. In addition, the forgings may be forged in hot, cold, warm or the like. In addition, the forging material may be one in which machining or the like has been performed.

Carburizing heat treatment

In the carburizing heat treatment step (S120), the forging material is subjected to carburizing heat treatment at 900 to 950 占 폚 for 1 to 2 hours.

At this time, the carburizing heat treatment may include a cracking process that cracks the surface texture under vacuum, a carburizing process in which carburizing gas is blown, and a diffusion process in which the burdened carburizing gas diffuses from the surface of the forging material. That is, the carburizing heat treatment needs to be performed in an atmosphere in which Si, Mn and Cr are selectively oxidized without oxidizing Fe, and more specifically, an inert gas containing H ₂ , H ₂ O, CO, CO ₂ , Atmosphere, or a low oxygen partial pressure atmosphere which is a mixed atmosphere thereof.

In this step, when the carburizing heat treatment temperature is lower than 900 캜, productivity is deteriorated due to an increase in the carburizing heat treatment time. On the other hand, if the carburizing heat treatment temperature exceeds 950 ° C, the mechanical properties of the steel after carburization may be deteriorated due to recrystallization.

In particular, if the carburizing heat treatment time is less than one hour, the carburization may be insufficient. On the other hand, if the carburizing heat treatment time exceeds 2 hours, carburization may occur to an excessively deep temperature and the toughness of the center portion of the forging material may be lowered.

Oil quenching

In the oil quenching step (S130), the forged material subjected to the carburizing treatment is oil quenched. Such oil quenching is performed for the purpose of preventing the surface hardness, which has been increased by the carburizing heat treatment, from lowering.

In this step, oil quenching is preferably carried out by dipping the forged carburized heat treated material in an oil-filled oil bath maintained at a temperature of 80 to 120 ° C. At this time, if the oil temperature is lower than 80 캜, the retained austenite is hardly present in the structure, and it may be difficult to secure toughness. On the other hand, if the oil temperature exceeds 120 캜, the formation of retained austenite during quenching increases, and the fatigue characteristic is likely to decrease.

Tempering

In the tempering step S140, the oil quenched forging is tempered. Such tempering is carried out for the purpose of alleviating the embrittlement of the carburized steel.

At this time, the tempering is preferably performed at 120 to 250 ° C for 1 to 3 hours. If the tumpering temperature is less than 120 ° C or the tempering time is less than 1 hour, it may be difficult to exhibit the brittle relaxation effect properly. Conversely, if the tempering temperature exceeds 250 DEG C or if the tempering time exceeds 3 hours, the surface hardness may be lowered.

The carburized steel produced in the above steps S110 to S140 increases the content of chromium (Cr) in the alloy components to 3.00 to 5.50 wt%, thereby promoting the generation of carbide with a carbon content of 1.0 wt% or less, Can be shortened.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Preparation of specimens

Specimens according to Examples 1 to 2 and Comparative Examples 1 and 2 were prepared with the composition shown in Table 1 and the process conditions shown in Table 2.

[Table 1] (unit:% by weight)

[Table 2]

2. Carbling Properties

Table 3 shows mechanical properties and carburization properties of the specimens according to Examples 1 and 2 and Comparative Examples 1 and 2.

[Table 3]

Referring to Tables 1 to 3, it was confirmed that the specimens according to Examples 1 and 2 exhibited excellent physical properties in terms of tensile strength and yield strength in comparison with Comparative Example 1, Respectively.

In the case of the specimens according to Examples 1 and 2, the carburization time was remarkably decreased with superior physical properties in terms of tensile strength and yield strength as compared with Comparative Example 2. [ In the case of the specimens according to Examples 1 and 2, although the content of carbon was significantly lower than that of Comparative Example 2, the addition of chromium at a high concentration promoted the formation of fine carbides in the carburized layer Respectively.

3. Carburized fatigue

Table 4 shows the carburization fatigue test results for the specimens according to Examples 1 and 2 and Comparative Examples 1 and 2.

[Table 4]

Referring to Tables 1 and 2 and Table 4, the specimens according to Examples 1 and 2 exhibited similar carburization fatigue characteristics to the specimens according to Comparative Example 2. Particularly, in the case of the specimens according to Examples 1 and 2, it can be confirmed that the contact fatigue increased about twice as much as that in Comparative Example 1.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Such changes and modifications are intended to fall within the scope of the present invention unless they depart from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

S110: Forging material preparation step
S120: Carburizing heat treatment step
S130: Oil quenching step
S140: Tempering step

Claims

(S): 0.05 to 0.8%, manganese (Mn): 0.4 to 1.5%, phosphorus (P): 0.015% or less, sulfur (S) : 0.001% or less of chromium (Cr), 0.005 to 0.040% of aluminum (Al), 0.005 to 0.040% of niobium (Nb), 0.001% or less of oxygen (O) % Or less and the remaining iron (Fe) and unavoidable impurities;
(b) carburizing the forging material at 900 to 950 캜 for 1 to 2 hours;
(c) oil quenching the carburized heat treated forging material; And
(d) tempering said oil quenched forging material. < RTI ID = 0.0 > 11. < / RTI >

The method according to claim 1,
In the step (a)
The forging material
Wherein at least one of molybdenum (Mo): 0.1 to 0.4%, boron (B): 0.001 to 0.004%, and titanium (Ti): 0.010 to 0.040% is further contained.

The method according to claim 1,
In the step (c)
The oil quenching
Wherein the carburizing treatment is performed by dipping the forged carburizing material in an oil-filled oil bath maintained at a temperature of 80 to 120 ° C.

The method according to claim 1,
In the step (d)
The tempering
At 120 to 250 占 폚 for 1 to 3 hours.

(P): 0.015% or less, sulfur (S): 0.010% or less, carbon (C): 0.10 to 0.30%, silicon (Si): 0.05 to 0.8%, manganese , 0.001 to 0.040% of niobium (Nb), 0.001% or less of oxygen (O), 0.015% or less of nitrogen (N), and 0.001 to 5% of chromium (Cr) (Fe) and unavoidable impurities,
A surface hardness of 60 HRC or more, and a core hardness of 490 Hv or more.

6. The method of claim 5,
The carburized steel
Wherein the molten steel further comprises at least one of molybdenum (Mo): 0.1 to 0.4%, boron (B): 0.001 to 0.004%, and titanium (Ti): 0.010 to 0.040%.