Steel containing high-density fine graphite nodules and preparation method thereof

Abstract

The invention discloses steel containing high-density small graphite nodules and a preparation method of the steel, and belongs to the technical field of metal materials and manufacturing. According to the preparation method disclosed by the invention, molten steel containing the graphite spheroidizing gene is firstly cast into a casting blank, then a casting blank sample is annealed to decompose carbides in the casting blank sample, the carbides are quickly decomposed into fine graphite nodules under the action of the graphite spheroidizing gene, and meanwhile, under the action of the graphite spheroidizing gene, the fine graphite nodules are formed. By means of the method, the purpose that high-density fine graphite phases are introduced into steel in an authigenic mode is effectively and efficiently achieved, and high-temperature solid solution C atoms are separated out into fine graphite nodules in the cooling process; the aluminum alloy die-casting die steel solves the problem that the existing aluminum alloy die-casting die steel does not contain a graphite phase with high thermal conductivity and high molten aluminum corrosion resistance, so that the aluminum alloy die-casting die steel is low in thermal conductivity and insufficient in molten aluminum corrosion resistance, and thus the aluminum alloy die-casting die steel premature failure is caused. The aluminum alloy die-casting die steel has great significance on the technical progress of aluminum alloy die-casting die and aluminum alloy die-casting production.

Description

Technical Field

[0001] The invention belongs to the technical field of metal materials and manufacturing, and in particular relates to a steel containing high-density fine graphite nodules and a preparation method thereof. Background Art

[0002] In recent years, with the vigorous development and wide application of the aluminum alloy industry, the demand for aluminum alloy die-casting molds has increased dramatically. These molds play a vital role in many fields such as automotive parts, aerospace, and electronic communications. At present, aluminum alloy die-casting molds are mainly manufactured by hot working die steel, which is widely used due to its high strength, good toughness and good wear resistance. However, hot working die steel has exposed some significant limitations in practical applications. Among them, low thermal conductivity is an issue that cannot be ignored. This makes it difficult for the mold to dissipate heat quickly during the die-casting process, causing heat accumulation, which in turn affects the overall performance of the mold and the quality of the die-casting. In addition, the resistance of hot working die steel to aluminum liquid corrosion is relatively poor. In the high temperature and high pressure die-casting environment, the mold surface is easily corroded by aluminum liquid, resulting in mold surface damage and performance degradation. These problems make the existing high-value aluminum alloy die-casting molds often fail prematurely due to insufficient thermal fatigue performance and aluminum liquid corrosion resistance, which not only increases the production cost of the enterprise, but also limits the production efficiency and quality improvement of aluminum alloy die-castings. Therefore, the research and development of new mold steel materials with higher thermal conductivity and stronger resistance to aluminum liquid corrosion is of great significance for preventing premature failure of aluminum alloy die-casting molds, extending their service life, and reducing production costs. In the future, with the continuous advancement of materials science and continuous innovation of technology, we have reason to believe that new materials that can overcome the limitations of existing mold steel materials will continue to emerge, bringing revolutionary changes to the aluminum alloy die-casting mold manufacturing industry.

[0003] A widely recognized fact about graphite materials and their products is that graphite, as a unique material, is considered a key component to improve mold performance due to its excellent performance characteristics. The high thermal conductivity of graphite means that it can transfer heat quickly, which is essential for the rapid heat dissipation of the mold during the die casting process. This feature helps to reduce heat accumulation and maintain the stability of the mold temperature, thereby improving the quality and production efficiency of die castings. In addition to high thermal conductivity, graphite also has good self-lubricating properties. This feature allows the mold to reduce the friction and wear caused by the die casting during the demoulding process during the die casting process, extending the service life of the mold. At the same time, the excellent corrosion resistance of graphite also makes it perform well in harsh die casting environments and is not easily corroded by aluminum liquid and other corrosive substances. More importantly, graphite has a non-wetting property with aluminum liquid. This means that during the die casting process, aluminum liquid is not easy to adhere to the graphite surface, thereby reducing the risk of damage to the mold surface and performance degradation. This feature plays a vital role in improving the mold's resistance to aluminum liquid corrosion. Based on these excellent properties of graphite, industry experts generally believe that the introduction of graphite phase into steel will significantly improve the thermal conductivity and corrosion resistance of steel. This will not only help to extend the service life of steel aluminum alloy die-casting molds, but also reduce production costs and improve production efficiency. Therefore, the research and development and application of mold steel materials containing graphite has become an important development trend in the aluminum alloy die-casting mold manufacturing industry. The existing methods of introducing graphite phase into steel are: 1. Introducing through external methods such as powder metallurgy. The graphite phase introduced by this method is coarse and unevenly distributed, which seriously deteriorates the mechanical properties of the steel; 2. High temperature annealing for up to 60-80 hours or even more than 100 hours. This method is inefficient for industrial production and is rarely used in practice.

[0004] Therefore, inventing an efficient method for preparing steel with a graphite phase is of great significance to industries such as aluminum alloy die-casting molds and aluminum alloy die-casting. Summary of the invention

[0005] The purpose of the present invention is to provide a steel containing high-density fine graphite nodules and a preparation method thereof, so as to solve the technical problem that the existing steel for aluminum alloy die-casting molds has low thermal conductivity and poor resistance to aluminum liquid corrosion due to the lack of graphite phase, which causes premature failure of aluminum alloy die-casting molds.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions: The invention discloses a method for preparing steel containing high-density fine graphite nodules, comprising the following steps: Casting a slab containing a spheroidizing gene; The ingot containing the spheroidizing gene is annealed and then cooled to obtain a steel containing high-density fine graphite nodules.

[0007] Furthermore, in terms of mass percentage, the components of the ingot containing the spheroidizing gene include: 1.5% to 2.5% C, 1.0% to 2.5% Si, ≤0.50% Mn, ≤0.08% P, 0.04% to 0.06% Mg 残留 , 0.01%~0.03%Re and ≤0.02%S, the balance is Fe and unavoidable impurities.

[0008] Furthermore, the casting process requires: the molten steel is prepared, smelted, inoculated and spheroidized according to the composition requirements of the steel, the spheroidizing gene is introduced into the molten steel, and then the molten steel with the spheroidizing gene is cast into a casting mold with strong cooling capacity to obtain a casting containing the spheroidizing gene.

[0009] Furthermore, the process parameters of the annealing treatment are: heating to 950-1050° C. at a heating rate of 300-500° C. / h and keeping warm for 10-20 hours.

[0010] Furthermore, the cooling treatment after the 950-1050°C insulation for 10-20h is as follows: firstly cooling to 830-860°C at a cooling rate of 40-60°C / h, and then cooling to room temperature for a second time.

[0011] Furthermore, the secondary cooling method is furnace cooling, air cooling, isothermal quenching or oil cooling.

[0012] The invention also discloses steel containing high-density fine graphite nodules prepared by the method.

[0013] Furthermore, in the structure of the steel containing high-density fine graphite nodules, the diameter of the graphite nodules is less than 50 um.

[0014] Furthermore, in the structure of the steel containing high-density fine graphite nodules, the diameter of the graphite nodules is 20-30 um.

[0015] Furthermore, the metal matrix containing high-density fine graphite nodular steel is a ferrite matrix, ferrite+pearlite, pearlite, austenite structure or martensite.

[0016] Compared with the prior art, the present invention has the following beneficial effects: The invention discloses a preparation method of steel containing high-density fine graphite nodules. Molten steel containing graphite spheroidizing genes is first cast into a casting billet, and then the casting billet sample is annealed to decompose carbides therein, and the carbides are quickly decomposed to form fine graphite balls under the action of the graphite spheroidizing genes; at the same time, under the action of the graphite spheroidizing genes, part of high-temperature solid-solution C atoms are precipitated into graphite balls during the heat preservation and cooling process. The method effectively and efficiently realizes the introduction of high-density fine graphite phases into steel, solves the problem that the existing aluminum alloy die-casting mold steel has low thermal conductivity and insufficient aluminum liquid corrosion resistance due to the lack of graphite phases with high thermal conductivity and high aluminum liquid corrosion resistance, thereby causing premature failure. The method has great significance for the technical progress of aluminum alloy die-casting molds and aluminum alloy die-casting production.

[0017] Furthermore, in the method, the steel material can be subjected to thermoplastic deformation like other steels before annealing to eliminate casting defects. By introducing spheroidizing genes during the solidification process, carbides and part of the solid solution C in the steel ingot can be quickly converted into fine high-density graphite nodules during the subsequent annealing process, thereby achieving effective and efficient preparation of steel containing high-density and fine graphite nodules. This is of great significance to steel manufacturing technology and its application. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The microstructure of the ingot containing the spheroidizing gene obtained by the present invention; Figure 2 This is the microstructure of the steel after annealing the ingot containing spheroidizing genes. DETAILED DESCRIPTION

[0019] In order to enable those skilled in the art to understand the characteristics and effects of the present invention, the following is a general description and definition of the terms and expressions mentioned in the specification and claims. Unless otherwise specified, all technical and scientific terms used in the text are the common meanings understood by those skilled in the art for the present invention. In the event of a conflict, the definition in this specification shall prevail.

[0020] The theories or mechanisms described and disclosed herein, whether correct or incorrect, should not limit the scope of the present invention in any way, that is, the present invention can be implemented without being limited by any specific theory or mechanism.

[0021] In this article, all features such as values, quantities, contents and concentrations defined in the form of numerical ranges or percentage ranges are for simplicity and convenience only. Accordingly, the description of numerical ranges or percentage ranges should be considered to have included and specifically disclosed all possible secondary ranges and individual values ​​within the range (including integers and fractions).

[0022] In this document, unless otherwise specified, “includes,” “including,” “contains,” “has,” or similar terms cover the meanings of “consisting of” and “mainly consisting of,” for example, “A includes a” covers the meanings of “A includes a and other” and “A only includes a.”

[0023] In this document, in order to make the description concise, not all possible combinations of various technical features in various embodiments or examples are described. Therefore, as long as there is no contradiction in the combination of these technical features, the various technical features in various embodiments or examples can be combined arbitrarily, and all possible combinations should be considered to be within the scope of this specification.

[0024] The present invention provides a method for preparing steel containing high-density fine graphite nodules, comprising the following steps: Step 1: Casting a billet containing a spheroidizing gene; Step 2: The ingot containing the spheroidizing gene is annealed and then cooled to obtain a steel containing high-density fine graphite nodules.

[0025] Preferably, the components of the spheroidizing gene-containing ingot include, by mass percentage: 1.5% to 2.5% C, 1.0% to 2.5% Si, ≤0.50% Mn, ≤0.08% P, 0.04% to 0.06% Mg 残留 , 0.01%~0.03%Re and ≤0.02%S, the balance is Fe and unavoidable impurities.

[0026] Preferably, the casting process requirements are: according to the composition requirements of the steel, the molten steel is batched, smelted, inoculated and spheroidized according to the batching, smelting, inoculation and spheroidization processes of ordinary ductile iron, and the spheroidization gene is introduced into the molten steel; then the molten steel with the spheroidization gene after inoculation and spheroidization is cast into a mold with strong cooling capacity, so that the molten steel solidifies into a graphite-free structure. That is, during solidification, a small amount of C in the molten steel that exceeds the eutectic composition is basically precipitated as carbide, and there are few / basically no graphite balls in the solidified structure. The corresponding solidified structure is a metal matrix + a small amount of carbide. The microstructure after solidification is shown in the attached Figure 1 shown.

[0027] Preferably, the cast ingot contains the gene that makes C spheroidize: 0.04%~0.06%Mg 残留 , 0.01%~0.03%Re and ≤0.02%S.

[0028] Preferably, the process parameters of the annealing treatment are: first, the steel billet cast in step S1 is heated to 950-1050°C at a heating rate of 300-500°C / h and kept at this temperature for 10-20h, so that the carbides and part of the dissolved C atoms in the solidified structure are quickly and completely transformed into graphite nodules and metal matrix under the action of the spheroidizing gene, and the steel of metal matrix + graphite nodules is obtained; the corresponding microstructure is shown in the attached Figure 2 .

[0029] Preferably, the cooling treatment after the insulation at 950-1050°C for 10-20h is as follows: first, the sample is cooled to 830-860°C at a cooling rate of 40-60°C / h, and the subsequent cooling method can be different according to the metal matrix to be obtained at room temperature, such as ferrite matrix, ferrite+pearlite, pearlite, austempered structure, martensite, etc., and different cooling methods such as furnace cooling, air cooling, isothermal quenching, oil cooling, etc. can be used to cool to room temperature to finally obtain steel with the desired structure.

[0030] Preferably, after annealing and cooling to room temperature, the diameter of the graphite nodules in the steel is small (all less than 50 μm, most of them are around 20-30 μm, and as the annealing temperature decreases, the diameter of the graphite nodules will further decrease), and some of them still retain the trend of solidified carbide strips and forked morphologies (see attached Figure 2 (indicated by the arrow in the middle).

[0031] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0032] The following examples use conventional instruments and equipment in the art. The experimental methods in the following examples where specific conditions are not specified are usually carried out under conventional conditions or under conditions recommended by the manufacturer. The various raw materials used in the following examples are conventional commercial products unless otherwise specified, and their specifications are conventional specifications in the art. In the specification of the present invention and the following examples, unless otherwise specified, "%" means weight percentage, "part" means weight part, and ratio means weight ratio.

[0033] Example 1 A method for preparing steel containing high-density fine graphite nodules comprises the following steps: Step 1: Casting a billet containing a spheroidizing gene; Step 2: annealing the ingot containing the spheroidizing gene, and then cooling it to obtain a steel containing high-density fine graphite nodules; In step 1, the casting process requirements are as follows: according to the composition requirements of the steel, the molten steel is batched, smelted, inoculated and spheroidized according to the batching, smelting, inoculation and spheroidization processes of ordinary ductile iron, and the spheroidization gene is introduced into the molten steel. Then, the molten steel with the spheroidization gene after inoculation and spheroidization is cast into a mold with strong cooling capacity, so that there are few or almost no graphite nodules in the solidified structure, and the corresponding solidified structure is metal matrix + carbide; After step 1, the cast ingot contains the gene that makes C spheroidize: 0.04% Mg 残留 , 0.03%Re and ≤0.02%S; After step 1, the composition of the cast ingot includes the following components by mass percentage: 1.5% C, 2.5% Si, ≤0.50% Mn, ≤0.08% P, 0.04% Mg 残留 , 0.03%Re and ≤0.02%S, the balance is Fe and unavoidable impurities; The annealing process in step 2 is as follows: first, the ingot cast in step 1 is heated to 950°C at a heating rate of 300°C / h and kept at this temperature for 20 hours, so that the carbides and partially dissolved C atoms in the ingot are completely transformed into graphite nodules and metal matrix under the action of spheroidizing genes, thereby obtaining steel with metal matrix + graphite nodules; After being kept at 950℃ for 20h, the sample was first cooled to 830℃ at a cooling rate of 40℃ / h, and then furnace cooled to obtain ferrite matrix steel; After optimization, after annealing and cooling to room temperature in step 2, the diameter of graphite nodules in the steel is small (all less than 50um, most of them are around 20~30um, and as the annealing temperature decreases, the diameter of the graphite nodules will further decrease), and some of them still retain the trend of carbide strips and forked shapes in the ingot.

[0034] Example 2 A method for preparing steel containing high-density fine graphite nodules comprises the following steps: Step 1: Casting a billet containing a spheroidizing gene; Step 2: annealing the ingot containing the spheroidizing gene, and then cooling it to obtain a steel containing high-density fine graphite nodules; In step 1, the casting process requirements are as follows: according to the composition requirements of the steel, the molten steel is batched, smelted, inoculated and spheroidized according to the batching, smelting, inoculation and spheroidization processes of ordinary ductile iron, and the spheroidization gene is introduced into the molten steel. Then, the molten steel with the spheroidization gene after inoculation and spheroidization is cast into a mold with strong cooling capacity, so that there are few or almost no graphite nodules in the solidified structure, and the corresponding solidified structure is metal matrix + carbide; After step 1, the cast ingot contains the gene that makes C spheroidize: 0.06% Mg残留 , 0.03%Re and ≤0.02%S; After step 1, the composition of the cast ingot includes the following components by mass percentage: 2.5% C, 1.0% Si, ≤0.50% Mn, ≤0.08% P, 0.06% Mg 残留 , 0.01%Re and ≤0.02%S, the balance is Fe and unavoidable impurities; The annealing process in step 2 is as follows: first, the sample cast in step 1 is heated to 1050°C at a heating rate of 500°C / h and kept at this temperature for 10 hours, so that the carbides and partially dissolved C atoms in the ingot are completely transformed into graphite nodules and metal matrix under the action of the spheroidizing gene, thereby obtaining a steel with metal matrix + graphite nodules; After being kept at 1050℃ for 10h, the sample was first cooled to 860℃ at a cooling rate of 60℃ / h, and then cooled to room temperature by air cooling, and finally a pearlite matrix steel was obtained; After optimization, after annealing and cooling to room temperature in step 2, the diameter of graphite nodules in the steel is small (all less than 50um, most of them are around 20~30um, and as the annealing temperature decreases, the diameter of the graphite nodules will further decrease), and some of them still retain the trend of carbide strips and forked shapes in the ingot.

[0035] Example 3 A method for preparing steel containing high-density fine graphite nodules comprises the following steps: Step 1: Casting a billet containing a spheroidizing gene; Step 2: annealing the ingot containing the spheroidizing gene, and then cooling it to obtain a steel containing high-density fine graphite nodules; In step 1, the casting process requirements are as follows: according to the composition requirements of the steel, the molten steel is batched, smelted, inoculated and spheroidized according to the batching, smelting, inoculation and spheroidization processes of ordinary ductile iron, and the spheroidization gene is introduced into the molten steel. Then, the molten steel with the spheroidization gene after inoculation and spheroidization is cast into a mold with strong cooling capacity, so that there are few or almost no graphite nodules in the solidified structure, and the corresponding solidified structure is metal matrix + carbide; After step 1, the cast ingot contains the gene that makes C spheroidize: 0.05% Mg 残留 , 0.02%Re and ≤0.02%S; After step 1, the composition of the cast ingot includes the following components by mass percentage: 2.0% C, 2.0% Si, ≤0.50% Mn, ≤0.08% P, 0.05% Mg 残留 , 0.02%Re and ≤0.02%S, the balance is Fe and unavoidable impurities; The annealing process in step 2 is as follows: first, the sample cast in step 1 is heated to 1000°C at a heating rate of 400°C / h and kept at this temperature for 15 hours, so that the carbides and partially dissolved C atoms in the ingot are completely transformed into graphite nodules and metal matrix under the action of the spheroidizing gene, thereby obtaining steel with metal matrix + graphite nodules; After being kept at 1000℃ for 15h, the sample was first cooled to 850℃ at a cooling rate of 50℃ / h, and then cooled to room temperature by oil cooling, and finally a martensitic steel was obtained; After optimization, after annealing and cooling to room temperature in step 2, the diameter of graphite nodules in the steel is small (all less than 50um, most of them are around 20~30um, and as the annealing temperature decreases, the diameter of the graphite nodules will further decrease), and some of them still retain the trend of carbide strips and forked shapes in the ingot.

[0036] The method of the present invention first casts high-C content molten steel containing graphite spheroidizing genes into a billet without graphite balls. Then, the billet is subjected to high-temperature annealing to decompose the carbides in the billet. During the carbide decomposition process, under the action of the graphite spheroidizing genes contained in the billet, the carbides are decomposed into spherical graphite instead of flocculent graphite, thereby realizing the transformation of the high-C, graphite-free billet into a steel material containing a high-density number of fine graphite balls through a solid-state phase change method. The steel prepared by the method has the characteristics of fine graphite balls, large number and short graphitization time, which will greatly improve its thermal conductivity, tribological properties and aluminum liquid corrosion resistance while retaining the excellent mechanical properties of the steel material, and provide technical support for the application of steel materials in modern industries such as aluminum alloy die-casting molds and self-lubricating transmission parts.

[0037] The present invention proposes an innovative method to successfully and efficiently introduce a high-density and fine graphite phase into steel. This breakthrough technology fundamentally solves the problem of low thermal conductivity and insufficient resistance to molten aluminum corrosion in existing aluminum alloy die-casting mold steel due to the lack of a graphite phase with high thermal conductivity and high resistance to molten aluminum corrosion. This innovation not only significantly improves the thermal conductivity efficiency of the mold steel and enhances its ability to resist molten aluminum corrosion, thereby effectively extending the service life of the aluminum alloy die-casting mold and reducing the cost loss caused by premature mold failure, but also has a milestone significance for promoting the overall progress of aluminum alloy die-casting molds and aluminum alloy die-casting production technology. The implementation of this method marks an important leap in the field of aluminum alloy die-casting mold material science, and injects strong impetus into the high-quality development of related industries.

[0038] The above contents are only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A method for preparing steel containing high-density fine graphite nodules, characterized in that: The following steps are involved: Casting a slab containing a spheroidizing gene; The ingot containing the spheroidizing gene is annealed and then cooled to obtain a steel containing high-density fine graphite nodules.

2. The method for preparing steel containing high-density fine graphite nodules according to claim 1, characterized in that: In terms of mass percentage, the components of the spheroidizing gene-containing ingot include: 1.5% to 2.5% C, 1.0% to 2.5% Si, ≤0.50% Mn, ≤0.08% P, 0.04% to 0.06% Mg 残留 , 0.01%~0.03%Re and ≤0.02%S, the balance is Fe and unavoidable impurities.

3. The method for preparing steel containing high-density fine graphite nodules according to claim 1, characterized in that: The casting process requirements are: batching, smelting, inoculating and spheroidizing the molten steel according to the composition requirements of the steel, introducing the spheroidizing gene into the molten steel, and then casting the molten steel with the spheroidizing gene into a casting mold with strong cooling capacity to obtain a casting billet containing the spheroidizing gene.

4. The method for preparing steel containing high-density fine graphite nodules according to claim 1, characterized in that: The process parameters of the annealing treatment are: heating to 950-1050° C. at a heating rate of 300-500° C. / h and keeping the temperature for 10-20 hours.

5. The method for preparing steel containing high-density fine graphite nodules according to claim 4, characterized in that: The cooling treatment after the 950-1050°C insulation for 10-20h is as follows: firstly cooling to 830-860°C at a cooling rate of 40-60°C / h, and then cooling to room temperature for a second time.

6. The method for preparing steel containing high-density fine graphite nodules according to claim 5, characterized in that: The secondary cooling method is furnace cooling, air cooling, isothermal quenching or oil cooling.

7. A steel containing high density fine graphite nodules, characterized in that: The method is prepared by any one of claims 1 to 6.

8. The steel containing high density fine graphite nodules according to claim 7, characterized in that: In the structure of the steel containing high-density fine graphite nodules, the diameter of the graphite nodules is less than 50 μm.

9. The steel containing high-density fine graphite nodules according to claim 7, characterized in that: In the structure of the steel containing high-density fine graphite nodules, the diameter of the graphite nodules is 20-30 um.

10. The steel containing high density fine graphite nodules according to claim 7, characterized in that: The metal matrix containing high-density fine graphite nodular steel is a ferrite matrix, ferrite+pearlite, pearlite, austenite or martensite.

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