Method for improving strength and toughness of cast steel based on scrap steel casting

Abstract

The invention relates to the technical field of metal metallurgy, in particular to a method for improving strength and toughness of cast steel based on waste steel casting, which prevents nitrogen from entering through oxidation-reduction reaction and whole-course argon blowing refining of molten steel, and adjusts components by adding a proper amount of alloy according to the components of the molten steel, so that the components of the molten steel after waste steel casting are improved, and the strength and toughness of the cast steel are improved. And then the cast steel with improved strength and toughness is obtained after cooling. The equipment adopted by the invention is simple, the quality of the cast workpiece can be improved and the product percent of pass can be increased without greatly changing the existing process, and the method has very good generalization performance.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgy, and in particular to a method for improving the strength and toughness of cast steel based on scrap steel smelting. Background Art

[0002] The raw materials used in molten steel smelting in the foundry industry are mostly scrap steel, including nozzles cut from castings, scraps from machinery factories, 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 causes more inclusions in the molten steel, making it difficult to ensure the strength and toughness of the cast workpieces. Summary of the invention

[0003] The purpose of the present invention is to provide a method for improving the strength and toughness of cast steel based on scrap steel smelting and casting, which improves the composition of molten steel by oxidation and adding alloys, and prevents nitrogen in the air from penetrating into the molten steel during the smelting and pouring process, thereby improving the organizational composition and structure of the casting.

[0004] The technical solution adopted by the present invention to solve the technical problem is: a method for improving the strength and toughness of cast steel based on scrap steel smelting, the steps are as follows: the first step is to melt steel into molten steel in a medium frequency furnace, and continuously add material to make the molten steel reach 2 / 3 to 3 / 4 of the furnace volume. During this period, an oxygen lance is inserted into the molten steel and oxygen is sprayed at a distance of 20 to 30 cm from the bottom of the furnace, and the oxygen pressure is 0.8 to 1.2 MPa. At the same time, slag-making agent is continuously added, and the smelting temperature is greater than 1600°C; the second step is, Remove the oxygen lance, blow argon from the bottom of the furnace, set aside the slag and sample the molten steel from at least 10 cm below the liquid surface, and perform spectral analysis on the chemical composition of the molten steel; the third step is to first add a carburizer to the molten steel, and then add aluminum particles; the fourth step is to add alloys to the molten steel to adjust the composition; the fifth step is to first add a carburizer to the bottom of the ladle when pouring the molten steel from the medium frequency furnace into the ladle, place slag cotton at the furnace mouth, add aluminum powder to the molten steel in the ladle after the molten steel is poured into the ladle, and place slag cotton to cover the liquid surface of the molten steel in the ladle.

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

[0006] Preferably, in the second to fifth steps, the melting temperature is 1530-1540°C.

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

[0008] Preferably, the recarburizer in the third step and the fifth step includes wood charcoal powder and coke powder.

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

[0010] Preferably, the ladle mouth of the fifth step is provided with an argon blowing gun which can blow argon upward from all sides. The argon blowing gun is handheld and includes a long handle which is hollow inside and can be held by hand. An air valve is provided at the gripping part of the long handle. The end of the long handle is connected to the air source through a hose, and a U-shaped ring tube is provided at the front end of the long handle.

[0011] Preferably, in the fourth step, the alloy-adjusted molten steel is added, and the composition is as follows: 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%, and the balance is iron.

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

[0013] C: Carbon can form an iron-carbon alloy with iron during steel smelting. Depending on the carbon content, it can greatly affect the steel structure, and further affect the strength and toughness of the steel. The first step of high-temperature smelting + oxygen blowing in the present invention is mainly to remove impurities in scrap steel. Because the oxygen content in molten steel is high, it reacts with carbon and greatly reduces the carbon content in the molten steel. Therefore, a carburizer must be added in the third step. Because the carburizer must be added to the bottom of the ladle in the fifth step, the carburizer added in the third step must make the carbon content in the molten steel return to the specific gravity of medium carbon steel, that is, the carbon content is controlled to be 0.3-0.45% in this step.

[0014] Al: The main functions of aluminum in steel are: ① calming the molten steel to prevent bubbles from forming when the molten steel solidifies; ② forming dispersed AlN particles, fixing nitrogen in the steel, and controlling the austenite grain size when the steel is reheated; ③ increasing the grain coarsening temperature, reducing the overheating sensitivity and hardenability of the steel, and improving the welding performance of the steel; ④ inhibiting the aging characteristics of low-carbon steel; ⑤ reducing the notch sensitivity and tough-brittle transition temperature of the steel. More importantly, aluminum reacts with oxygen to form aluminum oxide, which can strongly deoxidize the molten steel. For example, aluminum reacts with iron oxide to generate aluminum oxide with a high melting point and low density. However, adding too much aluminum will promote the graphitization of steel and reduce the high-temperature strength and toughness of steel. Therefore, the aluminum of the present invention mainly comes from smelting scrap steel, and the aluminum particles added in the third step are mainly for strong deoxidation, and the aluminum particles added in the fifth step are for nitrogen fixation, and the amount is very small. Finally, aluminum forms solid solution aluminum in the molten steel, and the content is not more than 0.01%.

[0015] Mo: Molybdenum has solid solution strengthening and creep resistance effects on ferrite, can improve the hardenability of steel, thereby improving the strength, hardness and thermal strength of steel, and can improve the tempering stability of steel, but if the molybdenum content exceeds 3%, the oxidation resistance of steel will be reduced. Therefore, the present invention controls the molybdenum content to 0.8-2%.

[0016] Nb: Niobium can greatly improve the yield strength of steel, control the cooling rate of molten steel, and induce precipitation to achieve dispersed distribution of precipitates, thereby enhancing the strength and toughness of steel. However, niobium is relatively expensive. The present invention controls the niobium content to 0.01-0.035%.

[0017] Mn: Manganese can improve the hardenability of steel and can improve the strength of steel through solid solution strengthening without affecting the plasticity and toughness of steel. In addition, manganese also reacts with oxygen and sulfur, thereby deoxidizing and desulfurizing. In view of the fact that manganese is relatively cheap, the manganese content is controlled at 1-2% in the present invention.

[0018] Cu, Sn: Copper, especially tin, can cause segregation of steel structure, but copper can improve the strength of steel, especially the yield strength ratio, and can also improve the room temperature impact toughness and fatigue strength of steel. In combination with phosphorus, it can also improve the corrosion resistance of steel. Therefore, the present invention controls the copper content to 0.1-0.8%, and the tin content to 0.015-0.03%, wherein the tin content comes from scrap steel and should be reduced as much as possible, and the copper content mainly comes from scrap steel, and can be added as appropriate in the fourth step.

[0019] Si: Silicon can significantly improve the strength and hardness of steel and increase the heat treatment temperature of steel, but silicon will strongly promote the graphitization of carbon. The silicon of the present invention mainly comes from scrap steel, and slag-forming agent quartz sand can be added in small amounts as needed to control the silicon content in molten steel to 0.1-0.5%.

[0020] Ca: Adding calcium to steel can refine the grains, partially desulfurize, and change the composition, quantity and form of non-metallic inclusions. It is basically similar to the effect of adding rare earth to steel. It can improve the corrosion resistance, wear resistance, high temperature resistance and low temperature resistance of steel; improve the impact toughness, fatigue strength, plasticity and welding performance of steel; increase the cold heading, shock resistance, hardness and contact endurance strength of steel. Adding calcium to cast steel greatly improves the fluidity of molten steel; the surface finish of the casting is improved, and the anisotropy of the structure in the casting is improved; its casting performance, thermal cracking resistance, mechanical properties and cutting performance are all increased to varying degrees. The calcium in the present invention mainly comes from the slag-making agent lime, and the calcium content is finally controlled to be no more than 0.01%.

[0021] Mg: Magnesium can react with oxygen and sulfur to deoxidize and desulfurize, which can improve the fluidity of steel and reduce the viscosity of slag. Therefore, the present invention adds dolomite to the slag-making agent, which can not only make slag, but also desulfurize and dephosphorize. 0.01-0.05% magnesium in molten steel is residual magnesium compound.

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

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

[0024] The beneficial effects of the present invention are: a method for improving the strength and toughness of cast steel based on scrap steel smelting and casting, which improves the composition of molten steel after scrap steel smelting and casting through redox reaction, argon blowing and refining of molten steel throughout the process to prevent nitrogen from entering, and adding alloys in appropriate amounts according to the composition of molten steel to adjust the composition, thereby improving the composition of molten steel after scrap steel smelting and casting, and then obtaining cast steel with improved strength and toughness after cooling. The present invention uses simple equipment, can improve the quality of cast workpieces and improve the product qualification rate without making major changes to the existing process, and has very good promotion. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0026] Description of reference numerals: 1——Long handle, 2——Air valve, 3——Ring tube. DETAILED DESCRIPTION

[0027] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments, but the implementation scope of the present invention is not limited thereto.

[0028] This embodiment mainly uses mechanical processing scraps and castings as scrap steel raw materials for casting molten steel smelting. During smelting, a large amount of scrap steel scraps are first added to the medium frequency furnace, and the smelting temperature is greater than 1600°C. After it is completely smelted, scrap steel and castings are added multiple times. Among them, the castings actually play a certain role in regulating the composition of molten steel. Starting from the second addition of scrap steel raw materials, it is necessary to use an oxygen lance to penetrate at least one meter below the molten steel liquid level or 20 to 30 cm from the bottom of the furnace to spray oxygen to speed up the scrap steel smelting speed and oxidize and remove impurities in the molten steel. In this step, in order to quickly remove impurities in the molten steel, such as N, P, S, tin and other elements, it is also necessary to add slag-making agents from the second addition of scrap steel. The slag-making agents use iron slag, lime and a small amount of dolomite.

[0029] When the molten steel in the medium frequency furnace reaches 2 / 3 to 3 / 4 of the furnace volume, the molten steel is refined to reduce oxygen and increase carbon, and improve the chemical composition of the molten steel. Therefore, in order to accurately control the amount of carburizer and various alloys added, the molten steel must first be tested for components. In this embodiment, the molten steel sample is sent by air for spectral analysis in the laboratory.

[0030] In order to avoid oxidation of molten steel in the furnace, which is very different from the sampled molten steel, the oxygen lance should be stopped from blowing oxygen at least ten minutes before the molten steel is sampled, and argon should be blown from the bottom of the furnace instead. The third step is to add a carburizer to the molten steel, which can be a carbon block or carbon powder, charcoal or coke. The carbon reacts with the free oxygen in the molten steel to reduce the oxygen content and increase the carbon content in the molten steel.

[0031] The fourth step, in this embodiment, the specific addition amounts of ferromolybdenum, ferroniobium and ferromanganese in the molten steel are calculated based on the molten steel composition detection in the second step and the weight of the molten steel in the furnace.

[0032] It should be noted that in order to control the composition of molten steel during refining and reduce the harm of molten steel to equipment, the melting temperature from the second step to the fifth step is 1530-1540℃.

[0033] When pouring molten steel from the medium frequency furnace into the ladle, the contact with air will increase. Therefore, the furnace mouth of the medium frequency furnace in this embodiment is arranged below the furnace mouth on the side of the furnace body. When the furnace body is tilted, the molten steel flows out from the furnace mouth, and the slag on the surface of the molten steel does not leak out. In addition, the furnace mouth is also provided with slag cotton to prevent the slag from flowing into the ladle.

[0034] When molten steel flows into the ladle, a large amount of air, especially oxygen and nitrogen, will be brought in. Therefore, in this embodiment, when pouring molten steel from the medium frequency furnace into the ladle, a recarburizer is first added to the bottom of the ladle. Here, the recarburizer is carbon powder. The carbon powder reacts with the oxygen in the molten steel to reduce the oxygen content of the molten steel on the one hand, and the gas generated on the other hand rises to prevent air, especially nitrogen, from being incorporated into the molten steel. After the molten steel is poured into the ladle, aluminum particles or aluminum powder are added to the molten steel in the ladle. Aluminum reacts with nitrogen in the molten steel to generate AlN. The melting point of AlN is higher than that of molten steel, and its density is lower than that of molten steel. It can float on the surface of the molten steel to achieve the effect of reducing nitrogen. Finally, slag cotton is placed on the surface of the molten steel in the ladle to cover and keep the molten steel warm.

[0035] In addition, in order to better prevent more air from being introduced into the molten steel when the molten steel is poured into the ladle, the embodiment Figure 1 An argon blowing gun capable of blowing argon upward from all sides is provided at the ladle opening. The argon blowing gun is handheld and comprises a long handle 1 with a hollow interior that can be held by hand. A gas valve 2 is provided at the gripping portion of the long handle 1. The end of the long handle 1 is connected to the gas source through a hose. A U-shaped ring tube 3 is provided at the front end of the long handle 1. In this way, an upward argon gas flow wall can be formed around the ladle opening to prevent air from flowing into the space between the furnace opening and the ladle.

[0036] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention, and for the convenience of describing the technical solution, the front, back, left, right, top, middle, bottom and other directions are set based on the drawings, rather than limiting the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A method for improving the strength and toughness of cast steel based on scrap steel smelting, characterized in that: The steps are as follows: the first step is to melt the steel material into molten steel in a medium frequency furnace, and continuously add material to make the molten steel reach 2 / 3 to 3 / 4 of the furnace volume. During this period, an oxygen lance is inserted into the molten steel and oxygen is sprayed at a distance of 20 to 30 cm from the bottom of the furnace, and the oxygen pressure is 0.8 to 1.2 MPa. At the same time, a slag-forming agent is continuously added, and the smelting temperature is greater than 1600°C; the second step is to remove the oxygen lance, blow argon from the bottom of the furnace, and sample the molten steel from at least 10 cm below the liquid surface, leaving the slag aside, and perform spectral analysis on the chemical composition of the molten steel; the third step is to first add a carburizer to the molten steel, and then add aluminum particles; the fourth step is to add alloys to the molten steel to adjust the composition; the fifth step is to first add a carburizer to the bottom of the ladle when pouring the molten steel from the medium frequency furnace, and place slag cotton at the furnace mouth. After the molten steel is poured into the ladle, aluminum powder is added to the molten steel in the ladle, and slag cotton is placed on the molten steel liquid surface of the ladle to cover it.

2. The method for improving the strength and toughness of cast steel based on scrap steel casting according to claim 1, characterized in that: From the second to the fifth step, argon gas is sprayed throughout the process to prevent oxygen and nitrogen from entering the molten steel.

3. The method for improving the strength and toughness of cast steel based on scrap steel casting according to claim 1, characterized in that: From the second step to the fifth step, the melting temperature is 1530~1540℃.

4. The method for improving the strength and toughness of cast steel based on scrap steel casting according to claim 1, characterized in that: The slag-making agents in the first step include iron slag, lime, quartz sand and dolomite.

5. The method for improving the strength and toughness of cast steel based on scrap steel smelting and casting according to claim 1, characterized in that: The recarburizers in the third and fifth steps include wood charcoal powder and coke powder.

6. The method for improving the strength and toughness of cast steel based on scrap steel smelting and casting according to claim 1, characterized in that: The alloys added in the fourth step include ferromolybdenum, ferroniobium, ferromanganese, and copper, and rare earth elements may be added as appropriate based on actual conditions.

7. The method for improving the strength and toughness of cast steel based on scrap steel smelting and casting according to claim 1, characterized in that: The ladle mouth of the fifth step is provided with an argon blowing gun which can blow argon upward from all sides. The argon blowing gun is handheld and includes a long handle which is hollow inside and can be held by hand. The gripping part of the long handle is provided with an air valve. The end of the long handle is connected to the air source through a hose, and the front end of the long handle is provided with a U-shaped ring tube.

8. The method for improving the strength and toughness of cast steel based on scrap steel casting according to claim 1, characterized in that: The fourth step is to add the alloy-adjusted molten steel, the composition of which is as follows: 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%, and the balance is iron.

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