KR20130046095A

KR20130046095A - Grain refined steel and method for manufacturing the same

Info

Publication number: KR20130046095A
Application number: KR1020110110453A
Authority: KR
Inventors: 김헌태; 신철수; 이선호; 권오철
Original assignee: 주식회사 일진글로벌
Priority date: 2011-10-27
Filing date: 2011-10-27
Publication date: 2013-05-07

Abstract

Fine grain steels and methods for their preparation are disclosed. The method for producing micro-grained steel according to the present invention includes carbon (C): 0.71 to 0.85, silicon (Si): 0.30 to 1.10, manganese (Mn): 0.20 to 1.50, and phosphorus (P): 0.02 by weight percent (wt%). Sulfur (S): 0.06 or less, Chromium (Cr): 0.25 or less, Molybdenum (Mo): 0.05 to 0.50, Nickel (Ni): 0.25 or less, Vanadium (V): 0.30 or less, Copper (Cu): 0.20 or less , Titanium (Ti): 0.003 or less, aluminum (Al): 0.005 to 0.055, boron (B): 0.001 to 0.005, oxygen (O): 0.0015 or less, providing a steel containing the remaining iron (Fe) and unavoidable impurities step; Heating the steel to an austenitization temperature; And quenching and tempering the austenitized steel, wherein the average grain size of the tempered steel is 5 μm or less.

Description

Fine grained steel and its manufacturing method {GRAIN REFINED STEEL AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to fine grained steel, and more particularly, to fine grained steel obtained by adding silicon, molybdenum and vanadium to fine grains to refine grains, and a method of manufacturing the same.

Steel applied to the outer ring and hub shaft of wheel bearings for automobiles is made of S55C class with 0.55 ~ 0.59% by weight of carbon, 0.15 ~ 0.30% by weight of silicon, 0.75 ~ 0.90% by weight of manganese (Mn). It is a pore stone. After hot-forging such a pore-steel, the surface is hardened by high-frequency heat treatment to secure wear resistance of tracks and major parts and to enhance rigidity.

Since the high-temperature heat treatment temperature is high, the high-temperature heat-treated bearing steel has a high probability of occurrence of poor quenching such as grain coarsening and poor cooling due to overheating. Heat treatment furnaces are difficult to satisfy.

The present invention is to provide a fine grained steel and a method for producing the fine grained steel by adding a grain refining alloying elements, such as silicon, molybdenum, vanadium, etc. in order to solve the above problems, and by refining the crystal grains by low temperature heat treatment as compared to the existing bearing steel The purpose.

Fine grain steel according to an embodiment of the present invention for achieving the above object by weight percent (wt%) carbon (C): 0.71 ~ 0.85, silicon (Si): 0.30 ~ 1.10, manganese (Mn): 0.20 ~ 1.50 , Phosphorus (P): 0.02 or less, sulfur (S): 0.06 or less, chromium (Cr): 0.25 or less, molybdenum (Mo): 0.05 to 0.50, nickel (Ni): 0.25 or less, vanadium (V): 0.30 or less, Copper (Cu): 0.20 or less, Titanium (Ti): 0.003 or less, Aluminum (Al): 0.005 to 0.055, Boron (B): 0.001 to 0.005, Oxygen (O): 0.0015 or less, remaining iron (Fe) and unavoidable impurities It includes.

The wheel bearing for automobiles according to the present invention is made of the fine grain steel.

In addition, the method for producing fine grained steel according to another embodiment of the present invention is carbon (C): 0.71 to 0.85, silicon (Si): 0.30 to 1.10, manganese (Mn): 0.20 to 1.50, by weight percent (wt%). Phosphorus (P): 0.02 or less, sulfur (S): 0.06 or less, chromium (Cr): 0.25 or less, molybdenum (Mo): 0.05 to 0.50, nickel (Ni): 0.25 or less, vanadium (V): 0.30 or less, copper (Cu): 0.20 or less, titanium (Ti): 0.003 or less, aluminum (Al): 0.005 to 0.055, boron (B): 0.001 to 0.005, oxygen (O): 0.0015 or less, and remaining iron (Fe) and unavoidable impurities Providing a comprising steel, heating the steel to an austenitizing temperature, and quenching and tempering the austenitized steel, wherein the average grain size of the tempered steel is 5 μm or less .

The austenitization temperature is characterized in that 750 ℃ ~ 950 ℃.

The tempering temperature is characterized in that 150 ℃ ~ 650 ℃.

According to another embodiment of the present invention, a method for manufacturing fine grain steel includes carbon (C): 0.71 to 0.85, silicon (Si): 0.30 to 1.10, manganese (Mn): 0.20 to 1.50, and phosphorus (wt%). P): greater than 0, 0.02% or less, sulfur (S): greater than 0, 0.06 or less, chromium (Cr): greater than 0, 0.25 or less, molybdenum (Mo): 0.05 to 0.50, nickel (Ni): greater than 0, 0.25 Vanadium (V): greater than 0 and less than 0.30, copper (Cu): greater than 0 and less than 0.20, titanium (Ti): greater than 0 and less than 0.003, aluminum (Al): 0.005 to 0.055, boron (B): 0.001 -0.005, oxygen (O): greater than 0 and less than or equal to 0.0015, providing a steel containing the remaining iron (Fe) and unavoidable impurities, and cooling the steel after high frequency heating and cooling the steel, the tempered steel The average grain size is 5 μm or less.

The wheel bearing for automobiles according to the present invention is manufactured by the above method.

According to the present invention, by adding a grain refining element to a high carbon steel base and performing heat treatment at low temperature, it is possible to provide a fine grained steel having an average grain size of 5 μm or less, and to improve durability by manufacturing wheel bearings for automobiles using such fine grained steel. Wheel bearings can be provided.

1 is a view showing the microstructure of the round bar manufactured from the ingot for fine grain steel according to the present invention.
FIG. 2 is a view showing the microstructure of the heat-treated part tempered after the high-frequency heat treatment of the round bar according to Figure 1 at 880 ℃.
FIG. 3 is a view showing the microstructure of the round bar of the round bar of FIG. 1 after heating the round bar at 840 ° C. for 90 minutes for quenching and quenching and heating at 180 ° C. for 120 minutes for cooling. .
FIG. 4 is a view showing the microstructure of the round bar tempered by heating the round bar according to FIG. 1 at 780 ° C. for 90 minutes and quenching with oil and then heating at 180 ° C. for 120 minutes.
5 is a graph comparing sizes of grains according to steel types and heat treatment conditions.
6 is a graph comparing hardness of steel and heat treatment conditions.

Advantages and features of the present invention, and methods of achieving the same will become apparent with reference to the embodiments described below in detail in conjunction with the accompanying drawings. However, it is to be understood that the present invention is not limited to the disclosed embodiments, but may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. It is intended that the disclosure of the present invention be limited only by the terms of the appended claims. Like reference numerals refer to like elements throughout the specification.

Hereinafter, with reference to the accompanying drawings it will be described with respect to the fine grain steel according to a preferred embodiment of the present invention and a method of manufacturing the same. In the following description, well-known functions or constructions are not described in detail to avoid unnecessarily obscuring the subject matter of the present invention.

The fine grained steel according to the present invention has a weight percent (wt%) of carbon (C): 0.71 to 0.85, silicon (Si): 0.30 to 1.10, manganese (Mn): 0.20 to 1.50, phosphorus (P): 0.02 or less, sulfur (S): 0.06 or less, chromium (Cr): 0.25 or less, molybdenum (Mo): 0.05 to 0.50, nickel (Ni): 0.25 or less, vanadium (V): 0.30 or less, copper (Cu): 0.20 or less, titanium ( Ti): 0.003 or less, aluminum (Al): 0.005 to 0.055, boron (B): 0.001 to 0.005, oxygen (O): 0.0015 or less, and the remaining iron (Fe) and inevitable impurities are included.

The amount of carbon (C) is from 0.71 wt% to 0.85 wt% or less. Carbon in the steel (C) is the element that is the fundamental element of the steel, the element that has the most influence on the mechanical properties of the steel, the austenite stabilizing element of the steel to form a martensite structure when quenched with an invasive solid solution. Increasing the amount of carbon improves the quenching hardness and strength, but adding too much increases the brittleness, lowers the elongation, induces deformation during quenching, and decreases the weldability of the steel, thereby significantly reducing the workability of the product. Limit to scope

The amount of silicon (Si) is 0.30 wt% to 1.10 wt% or less. Silicon is an unavoidable impurity remaining in pig iron and deoxidizer in steel, but it is a ferrite stabilizing element added as a deoxidizer during steelmaking. In the present invention, the austenite nucleation is promoted and added for the refinement of crystal grains, and when contained in a large amount in the steel, the toughness is lowered and the plastic workability is reduced, so the content is limited to the above-mentioned range.

The amount of manganese (Mn) is 0.20 wt% to 1.50 wt% or less. Manganese (Mn) is an impurity that is inevitably included in the steel, but is employed in the steel during the steel making process, and the rest is combined with sulfur (S) in the steel to exist in grains in the form of MnS, which is a nonmetallic inclusion. And improve the elongation. This MnS is ductile, and it extends | stretches along the process direction at the time of plastic working. The formation of MnS reduces the amount of sulfur (S) in the steel, thereby inhibiting the formation of FeS, a weak and low melting compound formed at grain boundaries. Manganese (Mn) improves the yield strength of carbon steel by increasing the fineness of pearlite and ferrite solid solution, and increases the depth of hardening when quenching, but when it contains a large amount, it causes quenching cracking or deformation. Limited to the above range.

The amount of phosphorus (P) is 0.02 wt% or less. Phosphorus (P) is an unavoidable impurity contained in the steel, and if it is uniformly distributed in the steel, there is no problem, but usually combines with iron (Fe) to form a harmful compound of Fe ₃ P. The compound is very fragile and segregated so that it is not homogenized even when subjected to annealing treatment. Therefore, the content is controlled in the above-described range because it decreases hot workability during forging and rolling, lowers impact resistance, promotes temper brittleness, and reduces normal temperature impact value.

The amount of sulfur (S) is 0.06 wt% or less. Sulfur (S) is an unavoidable impurity contained in the steel, which combines with manganese (Mn) in the steel to form MnS non-metallic inclusions to improve the machinability of the steel, but when the amount of manganese (Mn) in the steel is not sufficient, iron (Fe) It is formed of very fragile FeS in combination with the mesh distribution in the grain boundary to reduce the tensile strength, elongation and impact value to inhibit hot and cold processing, so the content is limited to the above range.

The amount of chromium (Cr) is 0.25 wt% or less. Chromium (Cr) is added to improve the hardenability of the steel, to reduce the tempering brittleness, and to improve the corrosion resistance. However, the content of the chromium (Cr) is limited to the aforementioned range in consideration of the possibility of intergranular corrosion and increased production cost.

The amount of molybdenum (Mo) is 0.05 wt% to 0.50 wt% or less. Molybdenum in steel is effective in improving the hardenability, thereby imparting temper embrittlement resistance. In addition, molybdenum is a carbide forming element that suppresses grain growth and carbon diffusion due to the formation of fine molybdenum carbide preferentially at the grain boundary, and serves to increase grain coarsening and suppress grain growth during high frequency heat treatment and Q / T heat treatment. However, since the addition of a large amount impairs workability and is expensive, the content is limited to the above-mentioned range.

The amount of nickel (Ni) is 0.25 wt% or less. Nickel (Ni), together with copper (Cu) and manganese (Mn), is an austenite stabilizing element that refines the steel structure and is well employed in austenite or ferrite to strengthen the matrix. Corrosion resistance is improved by adding a small amount, but the content is limited to the aforementioned range due to deterioration of workability and increase in manufacturing cost when a large amount is added.

The amount of vanadium (V) is 0.30 wt% or less. Vanadium is an element added to refine the grains of the steel of the present invention, the carbide formation ability is large, forming a fine vanadium carbide to suppress the growth of grains to refine the final structure, the tempering softening resistance is improved, but exceeds 0.30wt% If the effect reaches the saturation state, it is limited to the aforementioned range.

The amount of copper (Cu) is 0.20 wt% or less. Copper (Cu), together with nickel (Ni) and manganese (Mn), increases austenite fraction as an austenite stabilizing element and improves corrosion resistance by adding a small amount, but it causes red brittleness when a large amount is added. Hot workability deterioration and manufacturing cost increase to limit the content to the above-mentioned range.

The amount of titanium (Ti) is 0.003 wt% or less. Titanium (Ti) has a strong affinity with any element such as oxygen, carbon, nitrogen, sulfur and hydrogen, and also has a strong carbide forming ability to refine the crystal grains, but oxides and carbonitride forms such as TiO ₂ , TiC, TiN when added in large amounts Its content as a non-metallic inclusion in the furnace steel acts as a starting point for cracking, so its content is limited to the aforementioned range.

The amount of aluminum (Al) is 0.005 wt% to 0.055 wt% or less. Aluminum (Al) is an alloy component added as a deoxidizer during steelmaking, but is present as a non-metallic inclusion when a large amount is added, and the content is limited to the above-mentioned range because it weakens steel.

The amount of boron (B) is 0.001 wt% to 0.005 wt% or less. Since boron (B) in steel has a large quenchable drainage, its effect decreases toward elements, vacancy steels, and super-vacuum steels which improve the quenchability due to the addition of a small amount, so that the content of the boron (B) in the carbon content of the present invention is in the aforementioned range. Limited to

The amount of oxygen (O) is 0.015 wt% or less. Since oxygen is hardly dissolved in iron, oxygen is mainly present in the oxide-based nonmetallic inclusions such as CaO, SiO ₂ , Al ₂ O ₃ , Cr ₂ O ₃ , and TiO ₂ . Particularly, D-based (mainly CaO + Al ₂ O ₃ ) -based oxides do not deform even after hot working, and deteriorate hot workability, and maintain the spherical shape at the time of casting, even after hot working, to the product as it is. The stress due to the difference of the stress acts as a cause of the stress concentration on the oxide-based nonmetallic inclusions, which acts as a starting point for the formation of voids and cracks around the nonmetallic inclusions, which adversely affects the fatigue life. In addition, if a large amount is present, the cause of abnormal structure during carburization, lowering of hardenability, and promotion of austenite grain growth by heating are controlled at 0.015 wt% or less in steel.

Except for the above-mentioned elements, the rest of the fine grained steel is made of iron (Fe) and other unavoidable impurities.

The method for producing fine grained steel according to the present invention is carbon (C): 0.71 to 0.85, silicon (Si): 0.30 to 1.10, manganese (Mn): 0.20 to 1.50, phosphorus (P): 0 in weight percent (wt%). Greater than 0.02, sulfur (S): greater than 0, 0.06 or less, chromium (Cr): greater than 0, 0.25 or less, molybdenum (Mo): 0.05 to 0.50, nickel (Ni): greater than 0, 0.25 or less, vanadium (V) ): Greater than 0 and less than 0.30, copper (Cu): greater than 0 and less than 0.20, titanium (Ti): greater than 0 and less than 0.003, aluminum (Al): 0.005 to 0.055, boron (B): 0.001 to 0.005, oxygen ( O): providing a steel that is greater than 0 and less than or equal to 0.0015 and containing the remaining iron (Fe) and unavoidable impurities; Heating the steel to an austenitization temperature; And quenching and tempering the austenitized steel, wherein the average grain size of the tempered steel is 10 μm or less.

The austenitization temperature is a method for producing fine grain steel, characterized in that 750 ℃ ~ 950 ℃.

In the present invention, it corresponds to a hypereutectoid steel having a high carbon content of steel, and the austenitization start temperature is relatively low in the Fe-C state diagram, and the transformation temperature due to the addition of an alloying element can be lowered. The nitriding temperature can be controlled in a low range.

The tempering temperature is characterized in that the 150 ℃ ~ 650 ℃ and after heating to a high temperature, the martensite structure inside the steel formed by quenching is changed to a tempered martensite structure by tempering relatively high strength It is possible to manufacture steel with improved toughness.

According to another embodiment of the present invention, a method for manufacturing fine grain steel includes carbon (C): 0.71 to 0.85, silicon (Si): 0.30 to 1.10, manganese (Mn): 0.20 to 1.50, and phosphorus (wt%). P): 0.02 or less, sulfur (S): 0.06 or less, chromium (Cr): 0.25 or less, molybdenum (Mo): 0.05 to 0.50, nickel (Ni): 0.25 or less, vanadium (V): 0.30 or less, copper (Cu ): 0.20 or less, titanium (Ti): 0.003 or less, aluminum (Al): 0.005 to 0.055, boron (B): 0.001 to 0.005, oxygen (O): 0.0015 or less, containing the remaining iron (Fe) and unavoidable impurities Providing a river; And cooling the steel after high frequency heating, wherein the average grain size of the tempered steel is 5 μm or less.

The wheel bearing for automobiles according to the present invention is made of fine grain steel produced by the above manufacturing method.

Hereinafter, the method for producing fine grained steel according to the present invention will be described in detail through examples.

Table 1 is a table showing the components of the fine grained steel prepared by adding the reference steel and the main alloying elements of the fine grained steel according to the present invention.

In order to manufacture a steel having a composition as shown in Table 1 below, the raw material of the purity of 99.99% C, 99.7% Fe, 99.5% Si, 99.95% Mo, 99.9% V, 99.9% Mn vacuum induction melting method ( Table 1 shows the steel components and reference steels according to the present invention dissolved in vacuum induction melting (VIM).

ingredient C Si Mo V Mn P S Ni Cr Cu Al Comparative example 0.70 0.24 - - 1.00 0.008 0.003 0.014 0.16 0.009 0.0251 Example 1 0.83 0.47 0.0015 0.16 1.14 0.011 0.006 0.007 0.0012 0.007 0.0089 Example 2 0.84 0.48 0.40 0.0014 1.14 0.013 0.006 0.011 0.0016 0.008 0.0085 Example 3 0.85 0.49 0.39 0.16 1.14 0.014 0.006 0.009 0.0027 0.007 0.0099

In the vacuum induction melting method, the capacity of the furnace was 50 kg, the degree of vacuum was 10 ^-2 torr, and the melting temperature was 1,400 ~ 1,500 ° C.

Dissolved ingots were segregated, and homogenizing annealing was performed to obtain homogeneous tissue. The conditions were as follows.

In the case of Example 1, after heating to 1,250 ℃ was maintained for 2 hours and then air-cooled, Examples 2 and 3 to which the carbide-forming element molybdenum is added, because of the slow diffusion rate was maintained for 3 hours after heating to 1,250 ℃.

The ingot subjected to the homogenization treatment was free forged to produce a round bar with a diameter of 70 mm, and then the surface oxidation scale was removed by surface treatment to prepare a round bar having a final diameter of 60 mm.

The microstructure, hardness, high frequency heat treatment (microstructure, grain size), quenching and tempering characteristics (microstructure, grain size) of the developed steel were evaluated.

Table 2 is a table comparing the hardness (Brinnel Hardness) of the round bar of the steel according to the present invention manufactured by the vacuum induction melting method and the reference steel.

Hardness measurement Reference steel Example 1 Example 2 Example 3 HRB 102.5 106.8 108.7 110.2

As shown in Table 2, it can be seen that the hardness tends to increase with the addition of alloying elements compared to the reference steel.

1 is a view showing the microstructure of the round bar and the reference steel prepared from the ingot for micro-grained steel according to the present invention prepared by the vacuum induction melting method. It can be seen that both the reference steel and the fine grained steel of the present invention form pearlite (Pearlite) microstructure.

FIG. 2 shows a microstructure in which austenite grains of tempered steel are exhibited after high-frequency heat treatment of a round bar, and the grains of alloys to which alloying elements are added are finer than those of reference steels.

FIG. 3 is a view showing the microstructure of steel tempered by quenching the round bar after heating at 840 ° C. for 90 minutes and quenching the oil and heating at 180 ° C. for 120 minutes. As shown in FIG. 3, it can be seen that as the type of grain refinement element added increases, the structure becomes finer.

Figure 4 is a view showing the microstructure of the steel tempered by quenching (quenching) the oil and then quenched by heating the steel at 780 ℃ for 90 minutes in order to confirm the low temperature heat treatment effect, and heated for 120 minutes at 180 ℃. It can be seen that the trend is similar to that in FIG.

Figure 5 is a graph comparing the grain size of the reference steel and the heat treatment conditions of the steel according to the present invention, it can be seen that the crystal grains of the steel by high frequency heat treatment and quenching / tempering to 1/2 level than the reference steel finer have. In addition, it can be seen that the steel material heat-treated at a relatively low temperature of 780 ℃ obtained fine grains than the steel heat-treated at 840 ℃.

Figure 6 is a graph comparing the hardness of the reference steel and the heat treatment conditions of the steel according to the present invention, it can be seen that the hardness of the steel with the alloying element added slightly compared to the reference steel.

As a result of the above, it is possible to obtain a fine grained steel by the addition of the alloying element according to the present invention, it is possible to obtain a high strength steel having fine grains by low temperature heat treatment ultimately pursued by the present invention.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, You will understand.

Accordingly, the embodiments described above are to be understood in all respects as illustrative and not restrictive. The scope of the present invention is shown by the following claims rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be construed as being included in the scope of the present invention. .

Claims

By weight percent (wt%), carbon (C): 0.71 to 0.85, silicon (Si): 0.30 to 1.10, manganese (Mn): 0.20 to 1.50, phosphorus (P): 0.02 or less, sulfur (S): 0.06 or less, Chromium (Cr): 0.25 or less, molybdenum (Mo): 0.05 to 0.50, nickel (Ni): 0.25 or less, vanadium (V): 0.30 or less, copper (Cu): 0.20 or less, titanium (Ti): 0.003 or less, aluminum (Al): 0.005 to 0.055, boron (B): 0.001 to 0.005, oxygen (O): 0.0015 or less, fine grain steel containing remaining iron (Fe) and unavoidable impurities.

By weight percent (wt%), carbon (C): 0.71 to 0.85, silicon (Si): 0.30 to 1.10, manganese (Mn): 0.20 to 1.50, phosphorus (P): 0.02 or less, sulfur (S): 0.06 or less, Chromium (Cr): 0.25 or less, molybdenum (Mo): 0.05 to 0.50, nickel (Ni): 0.25 or less, vanadium (V): 0.30 or less, copper (Cu): 0.20 or less, titanium (Ti): 0.003 or less, aluminum (Al): 0.005 to 0.055, boron (B): 0.001 to 0.005, oxygen (O): 0.0015 or less, providing a steel containing remaining iron (Fe) and unavoidable impurities;
Heating the steel to an austenitization temperature; and
Quenching and tempering the austenitic steel,
The method of claim 1, wherein the average grain size of the tempered steel is 5 µm or less.

The method of claim 2,
The austenitization temperature is a method for producing fine grain steel, characterized in that 750 ℃ ~ 950 ℃.

The method of claim 2,
The tempering temperature is a method for producing fine grain steel, characterized in that 150 ℃ ~ 650 ℃.

By weight percent (wt%), carbon (C): 0.71 to 0.85, silicon (Si): 0.30 to 1.10, manganese (Mn): 0.20 to 1.50, phosphorus (P): 0.02 or less, sulfur (S): 0.06 or less, Chromium (Cr): 0.25 or less, molybdenum (Mo): 0.05 to 0.50, nickel (Ni): 0.25 or less, vanadium (V): 0.30 or less, copper (Cu): 0.20 or less, titanium (Ti): 0.003 or less, aluminum (Al): 0.005-0.055, boron (B): 0.001-0.005, oxygen (O): 0.0015 or less, including the remaining iron (Fe) and unavoidable impurities; And
Cooling the steel after high frequency heating;
The method of claim 1, wherein the average grain size of the tempered steel is 5 µm or less.

An automotive wheel bearing made of fine grain steel according to claim 1.

An automotive wheel bearing made of fine grain steel produced by the manufacturing method of any one of claims 2 to 5.