A method for improving strength and toughness of cast steel based on scrap melting

Abstract

The present application relates to the technical field of metal metallurgy, and particularly relates to a method for improving strength and toughness of cast steel based on scrap steel melting and casting, which improves the composition of molten steel after scrap steel melting and casting through oxidation-reduction reaction, prevents nitrogen from entering through full-course argon blowing of the molten steel, and adjusts the composition by adding appropriate alloy according to the composition of the molten steel, so as to obtain cast steel with improved strength and toughness after cooling. The present application has very good popularization because the equipment is simple, the existing process does not need to be greatly changed, the quality of cast workpieces is improved, and the product qualification rate is improved.

Description

Technical Field

[0001] This invention relates to the field of metal metallurgy, and in particular to a method for improving the strength and toughness of cast steel based on scrap steel melting and casting. Background Technology

[0002] The raw materials used in steel smelting in the foundry industry are mostly scrap steel, including sprues cut from castings, scrap from machine shops, slag from steel mills, and even recycled scrap steel. This makes it difficult to control the composition of the smelted steel. In addition, the smelting equipment mostly uses medium-frequency electric furnaces, which results in more inclusions in the smelted steel, making it difficult to guarantee the strength and toughness of the cast parts. Summary of the Invention

[0003] The purpose of this invention is to address the shortcomings of existing technologies by providing a method for improving the strength and toughness of cast steel based on scrap steel melting and casting. This method improves the composition of molten steel by oxidation and the addition of alloys, and prevents nitrogen from the air from penetrating into the molten steel during melting and casting, thereby improving the microstructure and structure of the casting.

[0004] The technical solution adopted by this invention to solve its technical problem is: a method for improving the strength and toughness of cast steel based on scrap steel melting and casting, comprising the following steps: First, steel material is melted into molten steel in an induction furnace, with continuous addition of material until the molten steel reaches 2 / 3 to 3 / 4 of the furnace volume. During this process, an oxygen lance is inserted into the molten steel and oxygen is injected at a distance of 20 to 30 cm from the furnace bottom, with an oxygen pressure of 0.8 to 1.2 MPa. Simultaneously, a slagging agent is continuously added, and the melting temperature is >1600℃; Second, ... Remove the oxygen lance, blow argon gas from the bottom of the furnace, skim off the slag, take a sample of the molten steel from at least 10 cm below the surface, and perform spectral analysis on the chemical composition of the molten steel; the third step is to add a carbon raiser to the molten steel, followed by aluminum granules; the fourth step is to add alloy adjusting components to the molten steel; the fifth step is to add a carbon raiser to the bottom of the ladle when pouring the molten steel from the induction furnace into the ladle, place slag-blocking cotton at the furnace opening, add aluminum powder to the molten steel in the ladle after pouring it into the ladle, and cover the surface of the molten steel in the ladle with slag-blocking cotton.

[0005] Preferably, from the second to the fifth step, argon gas is injected throughout to prevent oxygen and nitrogen from entering the molten steel.

[0006] Preferably, the melting temperature for steps two through five is 1530–1540°C.

[0007] Preferably, the slag-forming agent in the first step includes iron slag, lime, quartz sand, and dolomite.

[0008] Preferably, the carbonizers used in the third and fifth steps include wood charcoal powder and coke powder.

[0009] Preferably, the alloys added in the fourth step include ferromolybdenum, ferroniobium, ferromanganese, and copper, and rare earth elements may be added as appropriate according to the actual situation.

[0010] Preferably, the ladle in the fifth step is equipped with an argon blowing gun that can blow argon gas upward from all sides. The argon blowing gun is handheld and includes a hollow, handheld handle. The grip part of the handle is equipped with a gas valve. The end of the handle is connected to the gas source through a hose. The front end of the handle is equipped with a U-shaped ring tube.

[0011] Preferably, in the fourth step, the alloy-adjusted molten steel has the following composition: C: 0.3-0.55%, Mo: 0.8-2%, Nb: 0.02-0.035%, Mn: 1-2%, Cu: 0.1-0.8%, Sn: 0.015-0.03%, Si: 0.1-0.5%, Re: 0.002-0.007%, Als≤0.01%, Ca≤0.01%, Mg:≤0.02%, N≤0.005%, P≤0.01%, S≤0.005%, with the balance being iron.

[0012] The fourth step, adding alloys to adjust the composition of the molten steel, is based on the following considerations:

[0013] C: Carbon can form iron-carbon alloys with iron in steelmaking. The carbon content significantly affects the microstructure of the steel, thus influencing its strength and toughness. The first step of this invention, high-temperature smelting followed by oxygen blowing, primarily removes impurities from the scrap steel. Because the molten steel has a high oxygen content, it reacts with carbon, greatly reducing the carbon content. Therefore, a carbon raiser is added in the third step. Since a carbon raiser is added to the bottom of the ladle in the fifth step, the carbon raiser added in the third step must restore the carbon content in the molten steel to the proportion of medium-carbon steel; that is, the carbon content is controlled at 0.3–0.45% in this step.

[0014] The main functions of aluminum in steel are: ① to calm molten steel and prevent the formation of bubbles during solidification; ② to form dispersed AlN particles, fixing nitrogen in the steel and controlling the austenite grain size during reheating; ③ to increase the grain coarsening temperature, reduce the steel's overheating sensitivity and hardenability, and improve its weldability; ④ to inhibit the aging characteristics of low-carbon steel; and ⑤ to reduce the steel's notch sensitivity and ductile-brittle transition temperature. More importantly, aluminum reacts with oxygen to form alumina, which strongly deoxidizes molten steel. For example, the reaction of aluminum with iron oxide produces alumina with a high melting point and low density. However, excessive aluminum addition can promote graphitization and reduce the steel's high-temperature strength and toughness. Therefore, the aluminum in this invention mainly comes from smelting scrap steel. The addition of aluminum particles in the third step is primarily for strong deoxidation, and the addition of aluminum particles in the fifth step is for nitrogen fixation, in very small quantities. Ultimately, aluminum forms a solid solution in the molten steel, with a content not exceeding 0.01%.

[0015] Molybdenum (Mo) has solid solution strengthening and creep resistance effects on ferrite, improving the hardenability of steel, thereby increasing its strength, hardness, and hot strength. It also improves the tempering stability of steel. However, if the molybdenum content exceeds 3%, it will reduce the oxidation resistance of the steel. Therefore, this invention controls the molybdenum content to be between 0.8% and 2%.

[0016] Niobium (Nb) can greatly improve the yield strength of steel, control the cooling rate of molten steel, and induce precipitation to achieve a dispersed distribution of precipitates, thereby strengthening the strength and toughness of steel. However, niobium is relatively expensive. In this invention, the niobium content is controlled at 0.01-0.035%.

[0017] Mn: Manganese can improve the hardenability of steel, enhance its strength through solid solution strengthening, and does not affect its plasticity and toughness. Furthermore, manganese reacts with oxygen and sulfur, thus enabling deoxidation and desulfurization. Given that manganese is relatively inexpensive, this invention controls the manganese content to be between 1% and 2%.

[0018] Cu and Sn: Copper, especially tin, can cause segregation in steel microstructure. However, copper can improve the strength of steel, particularly the yield strength ratio, as well as its room temperature impact toughness and fatigue strength. Combined with phosphorus, it can also improve the corrosion resistance of steel. Therefore, this invention controls the copper content to 0.1–0.8% and the tin content to 0.015–0.03%. The tin content, derived from scrap steel, should be minimized. The copper content, primarily derived from scrap steel, can be added as needed in the fourth step.

[0019] Si: Silicon can significantly improve the strength and hardness of steel and increase its heat treatment temperature, but it strongly promotes the graphitization of carbon. In this invention, the silicon is mainly derived from scrap steel, with a small amount of slag-forming agent, quartz sand, added as needed to control the silicon content in the molten steel at 0.1–0.5%.

[0020] Adding calcium (Ca) to steel refines grains, partially desulfurizes, and alters the composition, quantity, and morphology of non-metallic inclusions, similar to the effect of adding rare earth elements. It improves the corrosion resistance, wear resistance, and high- and low-temperature performance of steel; enhances its impact toughness, fatigue strength, plasticity, and weldability; and increases its cold heading ability, shock resistance, hardness, and contact creep strength. Adding calcium to cast steel significantly improves its fluidity; improves the surface finish of castings; and reduces the anisotropy of the microstructure within the casting. Its casting performance, resistance to hot cracking, mechanical properties, and machinability are all increased to varying degrees. In this invention, the calcium primarily comes from the slagging agent lime, with the final calcium content controlled to be no more than 0.01%.

[0021] Mg: Magnesium can react with oxygen and sulfur, thereby deoxidizing and desulfurizing, which can improve the fluidity of steel and reduce slag viscosity. Therefore, this invention adds dolomite to the slagging agent, which, in addition to slagging, can also desulfurize and dephosphorize. 0.01-0.05% magnesium in molten steel is residual magnesium oxide.

[0022] N, P, S: Nitrogen, phosphorus, and sulfur are harmful elements in steelmaking and need to be controlled. That is, the content of these elements in molten steel needs to be reduced during the steelmaking process. This invention controls N≤0.005%, P≤0.01%, and S≤0.005%.

[0023] As for elements such as lead, nickel, chromium, and cobalt that may be contained in scrap steel, no specific control is made based on the melting point of lead, the effects of the aforementioned elements on steel, and the purpose of this invention.

[0024] The beneficial effects of this invention are: a method for improving the strength and toughness of cast steel based on scrap steel melting and casting. This method utilizes redox reactions, argon blowing throughout the molten steel refining process to prevent nitrogen ingress, and the appropriate addition of alloys to adjust the composition of the molten steel, thereby improving the composition of the molten steel after scrap steel melting and casting. Upon cooling, this results in cast steel with improved strength and toughness. The equipment used in this invention is simple, requiring no major changes to existing processes, yet it can improve the quality of cast workpieces and increase the product qualification rate, making it highly applicable and easily scalable. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the argon blowing gun of the present invention.

[0026] Explanation of reference numerals in the attached figures:

[0027] 1 – Long handle, 2 – Air valve, 3 – Ring pipe. Detailed Implementation

[0028] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the scope of the invention.

[0029] This embodiment primarily uses machining scraps and casting nozzles as raw materials for smelting foundry steel. During smelting, a large amount of scrap steel is first added to an induction furnace at a melting temperature >1600℃. After complete melting, scrap steel and casting nozzles are added multiple times. The casting nozzles effectively regulate the steel composition. From the second addition of scrap steel, an oxygen lance is used to blow oxygen at least one meter below the molten steel surface or 20-30 cm from the furnace bottom to accelerate the smelting speed and simultaneously oxidize and remove impurities from the molten steel. In this step, to quickly remove impurities such as N, P, S, and tin, a slagging agent is added starting from the second addition of scrap steel. The slagging agent consists of iron slag, lime, and a small amount of dolomite.

[0030] Once the molten steel in the intermediate frequency furnace reaches 2 / 3 to 3 / 4 of the furnace volume, refining begins to reduce oxygen and increase carbon content, thereby improving the chemical composition of the molten steel. Therefore, in order to accurately control the amount of carburizing agent and various alloys added, the composition of the molten steel must first be analyzed. In this embodiment, a sample of molten steel delivered by pneumatic conveying is used for spectral analysis in the laboratory.

[0031] To prevent oxidation of the molten steel in the furnace, which would result in a significant difference from the sampled molten steel, oxygen blowing from the oxygen lance must be stopped at least ten minutes before sampling, and argon blowing should be switched to the furnace bottom. The third step involves adding a carbon raiser to the molten steel. This can be carbon blocks or powder, charcoal, or coke. The carbon reacts with the free oxygen in the molten steel, reducing the oxygen content and increasing the carbon content.

[0032] In the fourth step, in this embodiment, ferromolybdenum, ferroniobium, and ferromanganese are added to the molten steel. The specific amount added is calculated based on the steel composition detection in the second step and the weight of the molten steel in the furnace.

[0033] It should be noted that, in order to facilitate the control of the steel composition during refining and to reduce the damage of molten steel to the equipment, the smelting temperature is 1530-1540℃ from the second to the fifth step.

[0034] When molten steel is poured from the induction furnace into the ladle, the contact with air is increased. Therefore, in this embodiment, the furnace opening of the induction furnace is set below the furnace opening on the side of the furnace body. When the furnace body is tilted, the molten steel flows out from the furnace opening and does not leak from the slag on the surface of the molten steel. In addition, slag-blocking cotton is also installed at the furnace opening to prevent slag from flowing into the ladle.

[0035] During the process of molten steel flowing into the ladle, a large amount of air, especially oxygen and nitrogen, is introduced. Therefore, in this embodiment, when pouring molten steel from the induction furnace into the ladle, a carbon raiser (carbon powder) is first added to the bottom of the ladle. The carbon powder reacts with the oxygen in the molten steel, reducing the oxygen content on one hand, and the resulting gas rises and prevents air, especially nitrogen, from dissolving into the molten steel. After the molten steel is poured into the ladle, aluminum granules or aluminum powder are added. The aluminum reacts with the nitrogen in the molten steel to form AlN. AlN has a higher melting point and lower density than molten steel, allowing it to float on the surface of the molten steel, thus achieving a nitrogen reduction effect. Finally, slag-blocking cotton is placed on the surface of the molten steel in the ladle to keep it warm.

[0036] In addition, to better prevent more air from being incorporated into the molten steel when it is poured into the ladle, this embodiment, for example... Figure 1 An argon-blowing gun is installed at the ladle inlet to blow argon gas upwards from all sides. This argon-blowing gun is handheld and includes a hollow, handheld handle 1. A gas valve 2 is installed at the gripping part of the handle 1. The end of the handle 1 is connected to a gas source via a flexible hose, and a U-shaped ring tube 3 is installed at the front end of the handle 1. This creates an upward wall of argon gas flow around the ladle inlet, preventing air from flowing between the furnace opening and the ladle.

[0037] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and for the convenience of describing the technical solutions, the front, back, left, right, top, middle, and bottom orientations are based on the accompanying drawings and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for improving the strength and toughness of cast steel based on scrap steel melting and casting, characterized in that: The steps are as follows: First, melt the steel in an induction furnace, continuously adding material until the molten steel reaches 2 / 3 to 3 / 4 of the furnace volume. During this process, insert an oxygen lance into the molten steel and blow oxygen from a distance of 20 to 30 cm from the bottom of the furnace at an oxygen pressure of 0.8 to 1.2 MPa. Simultaneously, continuously add slag-forming agents, maintaining a melting temperature >1600℃. Second, remove the oxygen lance and blow argon gas from the bottom of the furnace. Skim off the slag and take samples of the molten steel from at least 10 cm below the surface for spectral analysis of its chemical composition. Third, add a carburizing agent to the molten steel. Then, aluminum granules are added; in the fourth step, alloy adjustment components are added to the molten steel; in the fifth step, when pouring the molten steel from the induction furnace into the ladle, a carbon raiser is first added to the bottom of the ladle, slag-blocking cotton is placed at the furnace mouth, aluminum powder is added to the molten steel in the ladle after the molten steel is poured into the ladle, and slag-blocking cotton is placed to cover the surface of the molten steel in the ladle; from the second to the fifth step, argon gas is injected throughout the process to prevent oxygen and nitrogen from entering the molten steel, and the melting temperature is 1530~1540℃; the alloys added in the fourth step include ferromolybdenum, ferroniobium, ferromanganese and copper, and rare earth elements are added as appropriate according to the actual situation.

2. The method for improving the strength and toughness of cast steel based on scrap steel melting and casting according to claim 1, characterized in that: The slag-forming agent in the first step includes iron slag, lime, quartz sand, and dolomite.

3. The method for improving the strength and toughness of cast steel based on scrap steel melting and casting according to claim 1, characterized in that: The carbon raisers used in steps three and five include wood charcoal powder and coke powder.

4. The method for improving the strength and toughness of cast steel based on scrap steel melting and casting according to claim 1, characterized in that: The fifth step involves installing an argon blowing gun at the pouring ladle opening, which can blow argon gas upwards from all sides. This argon blowing gun is handheld and includes a hollow, handheld handle. The gripping part of the handle is equipped with a gas valve, and the end of the handle is connected to the gas source via a flexible hose. The front end of the handle is equipped with a U-shaped ring tube.

5. The method for improving the strength and toughness of cast steel based on scrap steel melting and casting according to claim 1, characterized in that: The fourth step involves adding alloy-modified molten steel with the following composition: C: 0.3–0.55%, Mo: 0.8–2%, Nb: 0.02–0.035%, Mn: 1–2%, Cu: 0.1–0.8%, Sn: 0.015–0.03%, Si: 0.1–0.5%, Re: 0.002–0.007%, Als≤0.01%, Ca≤0.01%, Mg:≤0.02%, N≤0.005%, P≤0.01%, S≤0.005%, with the balance being iron.

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