KR102096795B1 - Austempered alloy steel and method for manufacturing the same - Google Patents

Abstract

The present invention relates to an ostempered alloy steel made of a new structure and a method for manufacturing the same, wherein the ostempered alloy steel is a composite structure steel composed of only two structures, such as cementite ferrite and osperite, which can exhibit high mechanical properties corresponding to AHSS 3rd generation alloys. have.

Description

Ostempered alloy steel and its manufacturing method {AUSTEMPERED ALLOY STEEL AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to an ostempered alloy steel and a method for manufacturing the same, and more particularly, to a form of a new microstructure, and to an ostempford alloy steel that exhibits high strength and high ductility properties, and a method for manufacturing the same.

In the automotive field, the development of light and high-strength materials continues. In particular, the development of materials that simultaneously exhibit high strength and high ductility was an important task in the material science field.

Currently, the automotive industry is focusing on the development of advanced high strength steel (AHSS), which has both high tensile strength and elongation to improve collision safety, and classifies them into first, second, and third generations. The first-generation AHSS includes phases such as ferrite, austenite, pearlite, martensite, bainite, and the like, and consists of two or more complex phases to improve mechanical properties. The types include DP (dual phase) steel, CP (complex phase) steel, and TRIP (transformation induced plasticity) steel. As TRIP steel undergoes plastic deformation, metamorphic organic plasticity occurs in which residual austenite is transformed into martensite.

The 2nd generation AHSS material was developed to secure higher mechanical properties than the 1st generation, and the representative material is TWIP (twinning induced plasticity) steel, which is a kind of austenitic stainless steel. TWIP steel is an austenite that produces twins with deformation, and exhibits superior mechanical properties than TRIP steel. However, since the necessary additive elements such as aluminum, nickel, and manganese increase, the price competitiveness decreases, and there is a problem that noise or shaking occurs in the tensile curve due to the formation of twins during plastic deformation. Therefore, it is required to develop a third-generation AHSS material considering all aspects.

One object of the present invention is to provide an excellent elongation alloy and a manufacturing method.

A method of manufacturing an ostempered alloy steel for one purpose of the present invention comprises: a first step of heating a porosity steel comprising silicon and manganese to a first temperature below A ₃ transformation point and above A ₁ transformation point; And after the first step, the second step of quenching the heated ungseok steel at a second temperature above the first temperature of the first step and above the A ₀ transformation point.

In one embodiment, the porosity steel may include 0.02 to 0.7% by weight of carbon, 1.0 to 2.5% by weight of silicon, 0.1 to 1.0% by weight of manganese, and 87.9% by weight or more of iron.

In an embodiment, the first temperature of the first step may be 700 ° C to 870 ° C.

In one embodiment, the second temperature of the second step may be 200 ° C to 400 ° C.

In one embodiment, the first step may be performed for 30 minutes to 120 minutes.

In one embodiment, the second step may be performed for 10 minutes to 100 minutes.

Ostempered alloy steel for other purposes of the present invention comprises ferrite and osperite structures, produced through the manufacturing method of the present invention, and exhibits excellent mechanical properties.

In one embodiment, the ostempered alloy steel may be a mixture of osperite structures on a ferrite structure.

In one embodiment, the ostempered alloy steel may be one in which the ferrite is a base structure and the osperite structure is an island type.

In one embodiment, the ostempered alloy steel may exhibit tensile strength of 700 MPa or more and elongation of 30% or more.

In one embodiment, the ostempered alloy steel may be used as a lightweight alloy material.

In one embodiment, the ostempered alloy steel may be used as an automotive steel sheet.

Through the present invention, it is possible to provide an ostempered alloy steel made of a new structure and a method for manufacturing the same. Unlike the conventional alloys, the ostemperd alloy steel produced through the present invention is an alloy steel that forms a composite structure composed of only two structures: a cornerstone ferrite and a ferrite structure, which has excellent elongation, is lighter, and has a higher mechanical equivalent to a third-generation AHSS alloy. It can show properties.

1 is a view showing the structure of an ostempered alloy steel according to an embodiment of the present invention.
2 is a view showing a Fe-C state diagram.
Figure 3 is a schematic diagram showing a simple process for manufacturing an ostempered alloy steel according to an embodiment of the present invention.
4 and 5 are diagrams showing differences in tissue according to temperature and time.
6 is a view comparing tissues manufactured according to an embodiment.
7 is a view showing the results of a tensile test comparison.
8 and 9 are diagrams showing the tissue according to the ostempering temperature.
10 is a view showing a tensile graph according to the ostempering temperature.
11 is a view showing the results of X-ray diffraction analysis.
12 is a view showing a processing window section.

The terms used in the present application are only used to describe specific embodiments and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this application, the terms "include" or "have" are intended to indicate the presence of features, steps, actions, components, parts or combinations thereof described in the specification, one or more other features or steps. It should be understood that it does not preclude the existence or addition possibility of the operation, components, parts or combinations thereof.

Unless defined otherwise, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by a person skilled in the art to which the present invention pertains. Terms such as those defined in a commonly used dictionary should be interpreted as having meanings consistent with meanings in the context of related technologies, and should not be interpreted as ideal or excessively formal meanings unless explicitly defined in the present application. Does not.

The method of manufacturing an ostempered alloy steel of the present invention comprises: a first step of heating a porosity steel containing silicon and manganese to a first temperature below A ₃ transformation point and above A ₁ transformation point; And after the first step, a second step of quenching the heated ungolite steel at a second temperature below the first temperature of the first step and above the A ₀ transformation point.

In other words, the manufacturing method of the ostempered alloy steel comprises: a first step of heating an alloy steel containing silicon, manganese, carbon and iron to a temperature between A ₃ transformation point and A ₁ transformation point; And after the first step, a second step of rapidly cooling the heated alloy steel at a temperature lower than the heating temperature of the first step and higher than the A ₀ transformation point.

In other words, in the present invention, the alloy containing silicon, manganese, and carbon is heated to a high temperature for a certain time, and then quenched to a temperature lower than the heating to maintain a certain time, thereby including ferrite and osperite structures. It has been found that an ostempered alloy steel having excellent mechanical strength is produced.

The hypo-eutectoid steel of the present invention represents a steel type (alloy) containing carbon below the vacancy point, or means a steel containing 0.87% or less carbon. In addition, other elemental components may be added to the pore steel of the present invention, and impurities may be included.

1 is a view showing the structure of an ostempered alloy steel according to an embodiment of the present invention. Specifically, the microstructure of the ostempered alloy steel manufactured through the method of manufacturing the ostempered alloy steel of the present invention was confirmed through SEM. The dark area in FIG. 1 represents ferrite, and the brighter area represents osperite. As shown in FIG. 1, the osmford alloy steel produced through the manufacturing method of the present invention may exhibit a state in which the matrix structure is ferrite and osperite is formed on the ferrite. For example, the ferrite may be sparsely formed on the ferrite, or may be formed in an island shape. For example, it may be a mixture of ferrite and austenite.

The composite structure (base structure) made of austenite and ferrite is called ausferrite, and the shape of the osperite structure can be changed by a difference in the driving force and the carbon diffusion rate according to the change in the tempering temperature. .

2 is a view showing a Fe-C state diagram.

The alloy steel containing carbon below the vacancy point is an alloy steel containing 0.87% or less carbon, and is called hypo-eutectoid steel. In one embodiment, alloy steel containing 0.87% or less of carbon and manganese and silicon was used. Porosity steel represents alloy steel containing 0.85% or less of carbon.

The A ₃ transformation point is a temperature at which an A ₃ transformation occurs in a Fe-C state diagram, and may indicate a temperature at which an allotropic transformation occurs. A _{3 At the} transformation point, the body-centered cubic can be changed to a face-centered cubic, and the face-centered cubic can be changed to a body-centered cubic. The atomic arrangement or crystal lattice may be at a varying temperature, and may be a temperature that has an important effect in quenching, annealing, and soaking. The A ₃ transformation point may be 723 ° C to 910 ° C.

The A ₁ transformation point is a temperature at which an A ₁ transformation occurs in a Fe-C state diagram, and may indicate a temperature at which an eutectoid transformation occurs in the steel. At the A ₁ transformation point, austenite may change to ferrite and / or cementite, and ferrite and / or cementite may change to austenite. The A ₁ transformation point may be 700 ° C to 750 ° C.

The A ₀ transformation point is a temperature at which an A ₀ transformation occurs in a Fe-C state diagram, and may indicate a temperature at which magnetic transformation of cementite occurs. The A ₀ transformation point can also be referred to as the cementite curie point. The A ₀ transformation point may be 200 ° C to 220 ° C.

In one embodiment, the perforated steel may include various elements.

In one embodiment, the pore steel includes iron, carbon, silicon, and manganese, and other elemental components may be further included. For example, molybdenum, manganese, nickel, and the like may be included. In the case of using manganese, the difference in carbon concentration at the grain boundary is reduced by segregation of manganese, and accordingly, the growth rate of ferrite may be slowed down.

In one embodiment, the perforated steel may be an alloy steel comprising 0.02 to 0.7% by weight of carbon, 1.0 to 2.5% by weight of silicon, 0.1 to 1.0% by weight of manganese, and 87.9% by weight or more of iron.

In one embodiment, the perforated steel may be 0.1 to 0.5% by weight of carbon, 1.5 to 2.5% by weight of silicon, 0.1 to 0.5% by weight of manganese, and 90.0% by weight or more of iron and impurities. For example, the porosity steel may include 0.4% by weight of carbon, 2.3% by weight of silicon, 0.3% by weight of manganese, and 90.0% by weight or more of iron.

 In one embodiment, the perforated steel includes iron, carbon, silicon, and manganese, and may further include phosphorus and / or sulfur. For example, it may include 0.38% by weight of carbon, 2.32% by weight of silicon, 0.34% by weight of manganese, 0.0019% by weight of phosphorus, 0.0013% by weight of sulfur, and the balance of iron.

In one embodiment, the first temperature of the first step is below A ₃ transformation point and above A ₁ transformation point, for example, the second temperature may be 700 ° C to 900 ° C.

In one embodiment, the first temperature of the first step may be 700 ° C to 870 ° C. In one embodiment, the first step may be heating at 750 ° C to 850 ° C. For example, the first step may be to heat the pore steel comprising silicon and manganese to 800 ° C.

In one embodiment, the first step is 0.02 to 0.7% by weight of carbon, 1.0 to 2.5% by weight of silicon, 0.1 to 1.0% by weight of manganese, and 87.9% by weight of iron ore steel containing at least 87.9% by weight of A ₃ transformation point or less and A ₁ transformation point or more It may be a step of heating to temperature. For example, the first step is a step of heating an arsenic steel containing iron of 0.02 to 0.7% by weight of carbon, 1.0 to 2.5% by weight of silicon, 0.1 to 1.0% by weight of manganese, and 87.9% by weight of iron to 700 ° C to 900 ° C You can.

In one embodiment, the first step, the first step is 0.02 to 0.7% by weight of carbon, 1.0 to 2.5% by weight of silicon, 0.1 to 1.0% by weight of manganese, and 87.9% by weight of iron ore steel comprising at least 87.9% by weight of 750 ℃ to 850 It may be heated to ℃. For example, it may be to heat a porosity steel containing at least 0.4% by weight of carbon, 2.3% by weight of silicon, 0.3% by weight of manganese, and 90.0% by weight of iron to 800 ° C.

The microstructure of the perforated steel may be changed by heating in the first step. For example, austenitization may be carried out in the sub-hole steel. At this time, ferrite may be formed in the austenite structure by austenitization, and accordingly, the ferrite formed with the ferrite on the cornerstone ferrite and the austenite structure may be formed.

In one embodiment, the first step may be performed for 30 minutes to 120 minutes. For example, the first step may be performed for 50 minutes.

In one embodiment, the first step may be performed in an inert gas atmosphere. For example, the first step may be performed in an argon gas atmosphere.

In one embodiment, the second step may be quenching to 200 ℃ to 400 ℃. For example, the second step may be to quench to 300 ℃ to 400 ℃. For example, it may be to quench to 380 ℃ or 260 ℃. The second step may include a step of quenching the alloy heated in the first step, and osmosis may be performed. The ostempering may be separated from austenite into ferrite and carbide. In the present invention, it was confirmed that when the ostempering temperature is low, the amount of retained austenite decreases and the amount of ferrite increases to form a lower osperite in the hard tissue. In addition, carbides are precipitated inside the ferrite due to the low temperature, and ferrite, carbides and austenite structures inside the ferrite may appear, and when the austempering temperature is high, the amount of ferrite does not increase and the amount of austenite increases, so that the upper os It was confirmed that ferrite was formed. At this time, carbides did not precipitate, but excellent mechanical strength could not be expected.

In one embodiment, the second step may be performed for 10 minutes to 100 minutes. For example, the second step may be performed for 30 minutes.

In one embodiment, a box furnace in which an austenitizing furnace and an ostempering furnace are integrally formed may be used. For example, when the middle door is opened by using the integral furnace, it is possible to prevent a sudden decrease in temperature of the specimen, and the time required for quenching (quenching) with a salt bath while opening the furnace opening device is controlled within about 3 seconds. Is possible.

In the austenitization treatment of the present invention, a transfer device and stability are required to obtain a rapid cooling rate when quenched with an atmosphere and a tempering temperature for decarburization prevention and uniform temperature distribution during austenitization treatment.

In one embodiment, the tempering heat treatment may be performed by quenching and maintaining constant temperature at a cooling rate of about 180 ° C./min using a salt bath in a two-stage heat treatment furnace.

In one embodiment, the salt used for ostempering may be stable to temperature changes, and may have excellent cooling ability.

In one embodiment, a high heat transfer coefficient, no risk of explosion, and a low water content may be used for ostempering as a salt.

In one embodiment, NaNO ₃ and KNO ₃ may be mixed and used. For example, when mixing NaNO ₃ and KNO ₃ , the mass ratio of each may be 55:45.

Through the present invention, it is possible to manufacture an alloy steel formed only of ferrite (hematite ferrite) and osperite (composite structure comprising austenite and ferrite or complex structure in which ferrite structure is formed on austenite structure) not previously found. It was confirmed through the present invention that the osmford alloy steel made of ferrite and osperite was excellent in mechanical properties.

Ostempered alloy steel for other purposes of the present invention is produced through the manufacturing method of the present invention, includes ferrite and osperite structures, and exhibits excellent mechanical properties.

In one embodiment, the ostempered alloy steel may be a mixture of osperite structures on a ferrite structure.

In one embodiment, the ostempered alloy steel may be one in which the ferrite is a base structure and the osperite structure is an island type. Here, the island type does not mean that a certain shape is regular or repeated, but that the non-base tissue is formed in groups of nano or micro units on the base tissue. For example, by using ferrite as a matrix structure, it can mean that a fine osperite structure is formed in a group formed to a predetermined size, and is mixed on the matrix structure of ferrite.

In one embodiment, the austenite alloy steel may include a microstructure in which austenite is formed from ferrite as a matrix structure. In one embodiment, the austenite alloy steel may be an austenite structure formed on a ferrite structure and a ferrite formed on the austenite to form an austenite structure. Therefore, it may include a microstructure in the form of a mixture of ferrite and osperite. In one embodiment, the ostempered alloy steel may be formed of an island-type ferrite structure on the ferrite structure.

In one embodiment, the ostempered alloy steel is composed of only ferrite and osperite structures, and thus may exhibit excellent mechanical properties.

In one embodiment, the ostempered alloy steel may exhibit tensile strength of 700 MPa or more and elongation of 30% or more.

In one embodiment, the ostempered alloy steel may be used as a lightweight alloy material because of its high specific strength.

In one embodiment, the ostempered alloy steel may be used as a lightweight alloy material.

In one embodiment, the ostempered alloy steel may be used as an automotive steel sheet. In one embodiment, the ostemford alloy steel may be used as a lightweight material since there are few additional components. In one embodiment, the ostempered alloy steel may be used as an automotive steel sheet.

The crystal structure of austenite consists of a face centered cubic (FCC), and whether or not processing organic transformation can be determined by stacking fault energy (SFE). When the austenite has a lamination defect energy of 10 mJ / m ² or less, it may be transformed into ε martensite during processing. In addition, as the stacking fault energy increases, the microstructure may change to ε martensite, mechanical twins, dislocation cells, and the like. When the stacking defect energy is low below about 20 mJ / m ² , austenite may be transformed into ε martensite of hexagonal close packing (HCP) depending on the deformation, and again, body-centered cubic structure (BCC, body) Processed organic transformation, which can be transformed into centered cubic α 'martensite, can occur. However, when the lamination defect energy is 20 mJ / m ² or more, transformation from austenite to martensite may be suppressed, and strain twins may be formed. When the stacking fault energy is 40 mJ / m ² or more, austenite may undergo slip deformation due to dislocation. The stacking fault energy can be changed by adding elements such as aluminum, manganese, silicon, and carbon, and differences may occur in mechanical properties. For example, when carbon or aluminum is added, the phase defect of austenite to ε martensite can be suppressed by increasing the stacking fault energy.

The osmford alloy steel is formed of a ferrite and an osperite structure, thereby exhibiting high elongation and high strength characteristics.

In one embodiment, the ostempered alloy steel may exhibit tensile strength of 700 MPa or more and elongation of 30% or more. In one embodiment, the ostempered alloy steel may exhibit a tensile strength of 800 MPa and an elongation of 39%.

3 is a view briefly showing a process of manufacturing an ostempered alloy steel according to an embodiment of the present invention. As shown in FIG. 3, the method for manufacturing the ostempered alloy steel of the present invention can be roughly divided into two steps, and may include a heat treatment process of heating at a temperature of 700 ° C to 870 ° C and rapidly cooling to a temperature of 200 ° C to 400 ° C. have. The process of heating to 700 ° C to 870 ° C may be performed for 30 minutes to 120 minutes, and the process of quenching to 200 ° C to 400 ° C may be performed for 10 minutes to 100 minutes. Specifically, the manufacturing method of the present invention first has a section for raising the temperature, and then a section for maintaining the temperature at a high temperature. At this time, austenitization occurs, it is important to keep the temperature uniformly throughout the specimen and to spread the carbon evenly. Then there is a section to cool the temperature. At this time, it is important to cool the temperature quickly so that no pearlite is generated. The section that lowers and maintains the temperature is an ostempering section, which must be heat treated at a certain temperature or higher so that martensite is not formed, and the temperature must be maintained while waiting for osperite to be sufficiently formed. When the osperite is sufficiently formed, the specimen is taken out and cooled to room temperature.

Osteothermal heat treatment is divided into two stages, the first stage is austenitizing heat treatment, which makes the entire base tissue austenite (also referred to as the first stage osmosis reaction), and the second stage is rapidly cooled above Ms temperature to maintain constant temperature. It is divided into an ostempering heat treatment (also referred to as an ostemper two-step reaction).

In one embodiment, the heat treatment furnace may use a heat treatment furnace. For example, the ostemper heat treatment furnace may be a two-stage heat treatment furnace.

In one embodiment, an osmosis stage 1, that is, an atmosphere furnace may be used for austenitizing heat treatment, and for example, a method such as induction heating or flame may be used. In the present invention, the production of pearlite should be suppressed. Therefore, since it needs to be cooled quickly, a metal bath such as a salt bath, an oil bath, tin or molten lead, or forced hot air may be used. Since the oil bath deteriorates when the temperature exceeds 300 ° C, a salt bath in which KNO ₃ and NaNO ₃ are mixed at a constant ratio can be used.

In one embodiment, lowering the heating temperature of the first step, that is, the austenitization temperature, may prevent mechanical properties from being degraded by unmodified residual austenite, but may lower the carbon content of the matrix and reduce the hardenability. . Therefore, in the case of thick materials, an alloying element must be added or the austenitization temperature must be increased.

4 and 5 are diagrams showing differences in tissue according to temperature and time.

4 is a view showing the structure according to the temperature. Specifically, Figure 4 shows the appearance of the tissue according to the rapid cooling temperature of the second step of the manufacturing method of the present invention. Referring to FIG. 4, when the ostempering temperature decreases, a needle-like structure may be formed as shown in FIG. 4 a, and the amount of ferrite increases, but the amount of retained austenite decreases, thereby exhibiting low elongation at high strength. When the growth rate of the ferrite interface becomes faster than the carbon diffusion rate exiting the ferrite due to the low temperature, carbides may be precipitated inside the ferrite.

On the other hand, when the ostempering temperature is increased, it may have a feather-like structure as shown in FIG. 3B, and the amount of ferrite is reduced, but the amount of retained austenite is increased, thereby exhibiting low strength at high elongation. In addition, carbides do not precipitate at the austenite-ferrite interface due to the high carbon diffusion rate, but satisfactory mechanical properties cannot be expected in this case.

Therefore, only the manufacturing method of the present invention can produce a high-strength, high-elongation characteristic ostempered alloy steel.

5 is a view showing the mechanical properties over time. Specifically, Figure 5 shows the relationship with the mechanical properties over time of quenching in the second step of the manufacturing method of the present invention. When the ostempering temperature is low, it may be affected with time as shown in FIG. 5A. When the temperature is low, the diffusion rate of carbon decreases, so the overall reaction may be slowed down. Therefore, the lower the ostempering temperature is, the better strength can be obtained by heat treatment for a long time. When the ostempering temperature is higher than 395 ° C, a change in elongation with time may be large as shown in FIG. 5B. The high temperature increases the diffusion rate of carbon, so the two-step reaction can start from 60 minutes, carbides may precipitate in austenite, and the elongation may be greatly reduced. When the ostempering temperature is low, the elongation may be low up to 240 minutes and the change over time may not appear. This is because the rate of carbon diffusion decreases and carbon is not sufficiently diffused inside the austenite, so it is transformed into martensite.

A new alloy close to the 3rd generation AHSS that can solve the problems of the 1st and 2nd generations can be provided through the present invention.

Example 1 (preparation of perforated steel)

In accordance with an embodiment of the present invention, the ostempered steel has a carbon content lowered to 0.6% or less of carbon, 3.0.% Or less of silicon, 0.5% or less of manganese, and porosity steel containing iron and impurities in the balance (Fe-0.4C-2.3Si -0.3Mn steel) was used.

First, in order to prepare an ostempered alloy steel, the Fe-0.4C-2.3Si-0.3Mn steel was prepared as an ingot using a 50 kg vacuum induction furnace. The thickness of the ingot was 50 mm and reheated at 1250 ° C for 2 hours in a nitrogen atmosphere, and hot rolled to a thickness of 3 mm by 5 pass rolling at 890 ° C.

Production example (Ostempford alloy steel 1 and 2 production)

In the present invention, the austenitization temperature of the first step was adjusted to 800 ° C. lower than 900 ° C. in order to make the base tissue into a cornerstone ferrite and to form some osperite structures. At this time, argon gas was injected and performed for 50 minutes.

Then, in the second step, the ostempering was adjusted to 260 ° C and 380 ° C, respectively, and maintained isothermal for 30 minutes, followed by cooling to room temperature to prepare Production Example 1 (380 ° C) and Production Example 2 (260 ° C). .

When the austenitization heat treatment was performed at 900 ° C in the related art, the elongation tended to be slightly lowered because all the tissues were in the form of osperite. However, when the austenitization temperature was adjusted to 800 ° C. as in the embodiment of the present invention, a cornerstone ferrite was generated to improve the elongation.

The osmford alloy steel of the present invention was found to have a shape similar to that of a TRIP or DP steel in the form of a tensile curve during the manufacturing process or tensile deformation. It is similar that the structure before the austenite heat treatment is in a ferrite state with about 50% or more of austenite, but the TRIP steel precipitates a bainite phase, which is a fine carbide, while the austenite alloy steel of the present invention generates only ferrite during tempering. As a result, an osperite structure composed of a cornerstone ferrite and residual austenite and ferrite is formed.

Compared to TRIP steel, the heat treatment process to produce residual austenite is similar, but the reason for the difference in the structure can be explained from the difference in components. In the case of TRIP steel, austenite is stabilized by adding aluminum, copper, and manganese. In particular, manganese is an austenite stabilizing element, but it is an element that enhances the hardenability of austenite and promotes carbide. In addition, when austenitizing heat treatment is performed at a constant temperature to retain austenite at room temperature, a certain amount of cornerstone ferrite is produced and carbon is accumulated as austenite, but the carbon content in the retained austenite decreases again due to the formation of carbide during tempering. . However, in the case of the tempered alloy steel of the present invention, the carbon concentration of austenite is increased as ferrite is generated because the formation of carbide is suppressed in the tempering step due to the high silicon content. In addition, by controlling the tempering time, the heat treatment is not performed for a long time until the austenite is decomposed, so that a structure in which only ferrite and austenite is present is formed.

6 is a view comparing tissues manufactured according to an embodiment. Specifically, a of FIG. 6 is Fe-0.4C-2.3Si-0.3Mn steel used in one embodiment, b of FIG. 6 is Preparation Example 1 prepared according to one embodiment, and c of FIG. 6 is TRIP steel It is shown. Referring to FIG. 6, it can be considered that the osmford alloy steel and the TRIP steel of the present invention have similar phases such as gemstone ferrite and austenite, but since the method for generating residual austenite is different, the difference in austenite shape can be confirmed. have. Before the heat treatment, the structure (a) was composed of a cornerstone ferrite and pearlite, but after the heat treatment (b), it can be seen that all the pearlite structures are changed to an osperite structure. However, although the TRIP steel (c) has a similar shape, it is formed of bainite and austenite structures, not osperite, and it can be seen that all three are different.

7 is a view showing a tensile curve. It can be seen that the tensile curve of the ostempered alloy steel of the present invention is similar to that of TRIP steel as shown in FIG. 7. This is similar to the phenomenon that the phase transformation occurs from the retained austenite to martensite as the TRIP steel undergoes plastic deformation. However, even if the plastic deformation form appears similar, it cannot be said that the phase transformation from austenite to martensite of the austenford alloy steel of the present invention was made like TRIP steel.

The shape and mechanical properties of austenite may vary according to the difference in Ostemper temperature. It is common that the lower the tempering heat treatment temperature, the higher the strength and the lower the elongation. At this time, austenite also changes finely into a needle shape, and the phase fraction also changes. In general, it is known that the austenite fraction decreases when heat treatment is performed at a low temperature, and the carbon content in the austenite is also measured to be low. However, in this material, on the contrary, the lower the austempering temperature, the more coarse austenite was observed, and the difference between the carbon content and the stability in the austenite may affect mechanical properties. Therefore, the experiment was performed at 260 ° C and 320 ° C to compare and confirm the cause of strengthening of the ostempered alloy steel of the present invention.

In the present invention, various experiments were further performed to observe changes in microstructures, thereby confirming factors for strengthening the mechanical properties of the Ostempered alloy steel of the present invention.

8 and 9 are diagrams showing the tissue according to the ostempering temperature.

Specifically, FIG. 8 shows an optical microscope photograph before and after deformation according to the difference in ostempering temperature. FIG. 8A shows Preparation Example 2 before tensile test, FIG. 8B shows Production Example 1 before tensile test, and FIG. 8C shows tensile test. After Preparation Example 2, FIG. 8D shows an optical microscope image of Preparation Example 1 after a tensile test. In general, when ostempering at a high temperature, the osperite structure has a feathery shape, and at low temperature, it has a needle-like structure. However, in the ostempered alloy steel of the present invention, the primary ferrite is mainly formed, and since the osperite is expressed finely on an optical microscope magnification, the difference according to the ostempering temperature does not appear significantly. Even before and after the deformation, there was a change such as recrystallization under the ostempering 380 ° C condition, but it was difficult to confirm in detail under an optical microscope.

Therefore, in order to find out in more detail the shape difference of the osperite tissue, the difference was compared using a scanning electron microscope. Specifically, FIG. 9 shows before and after deformation according to the difference in ostempering temperature using a scanning electron microscope. FIG. 9A shows Production Example 2 before tensile test, and FIG. 9B shows Production Example 1 before tensile test, and FIG. 9C Is the preparation example 2 after the tensile test, and d in FIG. 9 shows the scanning electron microscope image of the preparation example 1 after the tensile test. Referring to FIG. 9, at 380 ° C. of ostempering, a ferrite structure composed of a cornerstone ferrite and a layered structure is formed as shown in FIG. 9B, and ferrite is also oriented in the interior of the osperite. In the condition of ostempering at 260 ° C, the cornerstone ferrite is the same, but osperite is shown in a coarse form, and it was confirmed that the structure thought to be ferrite inside the austenite is in the form of a needle. Even if the heat treatment time and the components are the same, the microstructure can be significantly changed only by the difference in the tempering temperature. However, in the post-transformation tissue, it appears that there is a greater change in the 380 ° C condition than the 260 ° C condition. As shown in d of FIG. 9, under the condition of 380 ° C., the entire layer of osperite is not disappeared, and some of them are agglomerated and partly torn. As shown in c of FIG. 9, in the condition of 260 ° C., agglomerates in the form of lumps that existed before deformation were formed in a torn form. Compared before and after the deformation, a larger change was observed in the tissue at 380 ° C. The reason may be that the specimen at 380 ° C. has twice the elongation and a larger amount of strain applied to the tissue than the specimen at 260 ° C.

10 is a tensile graph according to the ostempering temperature. When comparing the conditions of 260 ℃ and 380 ℃ for ostempering, it clearly shows how much the mechanical properties differ. The maximum tensile strength at 260 ° C. of ostempering was about 1,000 MPa, and the elongation was about 20%. The specimen used as a comparison object has a maximum tensile strength of about 800 MPa at 380 ° C. and an elongation of about 39%.

11 is a view showing the results of X-ray diffraction analysis. Specifically, FIG. 11 is an X-ray diffraction analysis result of Preparation Example 1 and Preparation Example 2 prepared through an embodiment of the present invention. It can be seen that the diffraction peaks of the austenitic (111) and (200) planes appear at a 2θ diffraction angle value of 43 ° (degrees) and 50 °, and thus the austenite structure generated after the austenite heat treatment is austenite. And ferrite.

As a result of backscattering electron diffraction analysis, the microstructure of osperite had austenite with one direction and ferrite with different directions inside. After deformation, the martensite was not observed in the area of osperite, but the area of non-analyzed (miss indexing) increased, and this can be seen as a phenomenon that the stress was concentrated in the area of osperite due to deformation.

12 is a view showing a processing window section. When the Fe-0.4C-2.3Si-0.3Mn steel used in one embodiment of the present invention is used to set the heat treatment time for osmosis to 30 minutes to 100 minutes, it enters the processing window section. Therefore, in the case of the present invention, the formation of carbide can be suppressed, and a material having excellent mechanical properties having only two phases of ferrite and austenite can be secured.

In general, the lower the austempering heat treatment temperature, the higher the strength and the lower the elongation. At this time, austenite also changes finely in a needle-like form, and the phase fraction also changes. In general, it is known that heat treatment at a low temperature reduces the fraction of austenite, and the carbon content in the austenite is also measured to be low.

However, in the case of the present invention, on the contrary, the lower the ostempering temperature, the more coarse austenite was observed, and it may be that excellent mechanical properties are exhibited by a difference in the carbon content and stability inside the austenite.

In conventional adi (austempered ductile iron), a spheroidal graphite structure is formed, and spheroidal graphite is a cause of alloy cracking, which adversely affects mechanical properties such as tensile strength and impact strength. Through the present invention, these problems can be solved, and an elongation is excellent and a lighter alloy material can be provided. In addition, since the amount of the other components added to the perforated steel used as a material is small, there is an effect of reducing costs.

In the ostempered alloy steel of the present invention, the microstructure under the condition of ostempering 380 ° C is a composite structure composed of a cornerstone ferrite and osperite. The austenite constituting the osperite has a very fine shape at a level of several tens of nm and forms a layered structure with ferrite, and is a structure supersaturated with carbon.

Although described above with reference to the preferred embodiment of the present invention, those skilled in the art may variously modify and change the present invention without departing from the spirit and scope of the present invention as set forth in the claims below. You will understand that you can.

Claims (12)

A _first for heating a porosity steel containing iron of 0.02 to 0.7% by weight of carbon, 1.0 to 2.5% by weight of silicon, 0 to 0.5% by weight of manganese and 87.9% by weight or more of iron to a first temperature below A ₃ transformation point and above A ₁ transformation point step; And
After the first step, the second step of quenching the heated ungseok steel to a second temperature below the first temperature of the first step and above A ₀ transformation point; includes,
Characterized in that it comprises a cornerstone ferrite and osperite structure,
Method of manufacturing Ostempered alloy steel.
delete According to claim 1,
The first temperature of the first step is characterized in that 700 ℃ to 870 ℃,
Method of manufacturing Ostempered alloy steel.
According to claim 1,
The second temperature of the second step is characterized in that 200 ℃ to 400 ℃,
Method of manufacturing Ostempered alloy steel.
According to claim 3,
The first step is characterized in that performed for 30 minutes to 120 minutes,
Method of manufacturing Ostempered alloy steel.
According to claim 4,
The second step is characterized in that performed for 10 minutes to 100 minutes,
Method of manufacturing Ostempered alloy steel.
Prepared through the manufacturing method of any one of claims 1 and 3 to 6,
Containing the cornerstone ferrite and osperite structure, showing excellent mechanical properties,
Ostempered alloy steel.
The method of claim 7,
The ostempered alloy steel is characterized in that the osperite structure is mixed on the base ferrite structure,
Ostempered alloy steel.
The method of claim 8,
The ostempered alloy steel is characterized in that the base ferrite is a base structure and the osperite structure is in an island type,
Ostempered alloy steel.
The method of claim 7,
The Ostempered alloy steel is characterized by exhibiting a tensile strength of 700 MPa or more and an elongation of 30% or more,
Ostempered alloy steel.
The method of claim 7,
The ostempered alloy steel is characterized in that it is used as a light alloy material of high specific strength,
Ostempered alloy steel.
The method of claim 11,
The Ostempered alloy steel is characterized in that it is used as an automotive steel sheet,
Ostempered alloy steel.

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