A cast steel part, its preparation method and application

Abstract

The present invention belongs to the technical field of metallurgical materials, and particularly relates to a cast steel component, a preparation method thereof and an application. The preparation method of the cast steel component provided by the present invention comprises the following steps: melting the raw materials for preparing the cast steel component to obtain a melt; mixing the melt, a deoxidizer and a refining agent for deoxidation refining and casting to obtain a blank casting; and performing heat treatment on the blank casting to obtain the cast steel component. By optimizing the chemical composition of the cast steel component, and simultaneously improving the purity of molten steel by means of a deoxidizer and a refining agent as well as optimizing the method and control parameters of heat treatment, the present invention can significantly improve the low-temperature impact toughness of the cast steel component cast by an intermediate frequency induction furnace, can meet the use safety of bridge construction under extremely cold climate conditions, and can reduce the production cost of the cast steel component, and reduce resource consumption and environmental pollution.

Description

Technical Field

[0001] The present invention belongs to the technical field of metallurgical materials, and particularly relates to a cast steel part, a preparation method thereof, and an application thereof. Background Art

[0002] Currently, bridge construction has become prevalent across the globe, with its footprint steadily extending to extreme climate regions such as permafrost and alpine areas (-60°C to 60°C). This trend has imposed more stringent requirements on the cold resistance performance of materials in bridge engineering. Among numerous cold resistance evaluation indicators, low-temperature impact toughness plays a crucial role, directly related to the structural safety and durability of bridges in extremely low-temperature environments. The optimization of this performance is deeply influenced by multiple factors such as chemical composition regulation, fine design of material microstructure, and improvement of molten steel purity.

[0003] To ensure that the molten steel reaches a highly pure state without pores and slag, traditionally, enterprises mostly use electric arc furnaces combined with complex refining processes to remove impurities with refined means, in order to obtain cast steel parts with relatively good low-temperature impact toughness as much as possible.

[0004] However, since the 1990s, with the rapid development of medium-frequency induction furnace technology, its advantages of high efficiency and environmental protection have gradually emerged, resulting in a reduction in the application scope of electric arc furnaces. Behind this transformation are the practical problems of large losses of electrodes and metal raw materials during the production process of electric arc furnaces, as well as relatively high emissions of harmful gases and particulate matter. However, due to the fact that medium-frequency induction furnace technology can only remove some impurities and cannot achieve the same refined impurity removal as electric arc furnace technology, the low-temperature impact toughness of the obtained cast steel parts is relatively poor, unable to meet the use safety requirements of bridge construction under extremely cold climate conditions. Summary of the Invention

[0005] The purpose of the present invention is to provide a cast steel part, a preparation method thereof, and an application thereof. The preparation method provided by the present invention is applicable to cast steel parts cast by medium-frequency induction furnaces, can significantly improve the low-temperature impact toughness of cast steel parts cast by medium-frequency induction furnaces, can meet the use safety requirements of bridge construction under extremely cold climate conditions, and can reduce the production cost of cast steel parts, as well as reduce resource consumption and environmental pollution.

[0006] To achieve the above purpose, the present invention provides the following technical solutions:

[0007] The present invention provides a preparation method of a cast steel part, comprising the following steps:

[0008] The raw materials for preparing the cast steel parts are melted to obtain a melt; the cast steel parts include the following elements in mass percentage: C 0.17 - 0.2%, Si 0.2 - 0.44%, Mn 1.1 - 1.3%, P ≤ 0.025%, S ≤ 0.025%, Cr 0.15 - 0.3%, Ni 0.3 - 0.5%, Mo ≤ 0.17%, Cu 0.15 - 0.45%, and the balance is Fe;

[0009] The deoxidizer and refining agent are placed at the bottom of the ladle, and the melt is poured into the ladle for deoxidation refining to obtain a refined melt;

[0010] The refined melt is cast to obtain a rough cast;

[0011] The rough cast is heat-treated; the heat treatment includes Method 1 or Method 2; Method 1 includes normalizing and the first tempering in sequence, and the holding temperature of the normalizing is 30 - 80°C above the AC3 temperature; Method 2 includes subcritical normalizing and the second tempering in sequence, and the holding temperature of the subcritical normalizing is greater than the AC1 temperature and less than the AC3 temperature.

[0012] Preferably, the melting is carried out in an intermediate frequency induction furnace; the tapping temperature of the melt is 1640 - 1680°C.

[0013] Preferably, the deoxidation refining is carried out by secondary refining outside the furnace;

[0014] The deoxidizer is a silicon-aluminum-barium-vanadium deoxidizer, and the mass ratio of the deoxidizer to the melt is 2.5 - 3.5‰; the residual aluminum content in the refined melt ≤ 0.05wt%;

[0015] The refining agent is a rare earth silicon alloy, and the mass ratio of the refining agent to the melt is 1 - 2‰.

[0016] Preferably, the deoxidation refining is carried out in the ladle, and the temperature in the ladle is not lower than 1620°C;

[0017] After obtaining the refined melt and before carrying out the casting, it further includes: standing the refined melt, the standing is carried out in the ladle, the standing time ≥ 5 min, and the standing temperature is not lower than 1600°C;

[0018] The casting temperature is not lower than 1580°C; the casting is carried out by bottom pouring.

[0019] Preferably, the AC3 temperature is 840 - 850°C, and the AC1 temperature is 720 - 740°C.

[0020] Preferably, the heating process of the normalizing treatment or the subcritical normalizing treatment includes: heating from room temperature to the temperature of the low-temperature section for heat preservation, and after the heat preservation ends, heating from the temperature of the low-temperature section to the temperature of the high-temperature section for heat preservation;

[0021] The heat preservation time at the temperature of the low-temperature section is 1 - 2 h;

[0022] The heating rate from the temperature of the low-temperature section to the temperature of the high-temperature section > 200 °C / h;

[0023] The heat preservation time at the temperature of the high-temperature section is 2 - 5 h.

[0024] Preferably, the heat preservation temperatures of the first tempering treatment and the second tempering treatment are independently 600 - 640 °C;

[0025] The heat preservation times of the first tempering treatment and the second tempering treatment are independently 6 - 8 h;

[0026] The cooling method after the normalizing treatment or the subcritical normalizing treatment is water mist cooling; the interval time between the first tempering treatment and the normalizing treatment ≤ 12 h; the interval time between the second tempering treatment and the subcritical normalizing treatment ≤ 12 h; the cooling methods of the first tempering treatment and the second tempering treatment are air cooling.

[0027] The present invention provides a steel casting prepared by the preparation method described in the above technical solution.

[0028] Preferably, the -40 °C impact toughness AK of the steel casting V2 is such that the average value ≥ 22 J and the single value ≥ 15 J.

[0029] The present invention provides the application of the steel casting described in the above technical solution in bridge engineering.

[0030] The present invention provides a method for preparing a cast steel component, comprising the following steps: melting the raw materials for preparing the cast steel component to obtain a melt; the cast steel component comprises elements with the following mass percentages: C 0.17 - 0.2%, Si 0.2 - 0.44%, Mn 1.1 - 1.3%, P ≤ 0.025%, S ≤ 0.025%, Cr 0.15 - 0.3%, Ni 0.3 - 0.5%, Mo ≤ 0.17%, Cu 0.15 - 0.45%, and the balance is Fe; placing a deoxidizer and a refining agent at the bottom of a ladle, pouring the melt into the ladle for deoxidation refining to obtain a refined melt; pouring the refined melt to obtain a blank casting; performing heat treatment on the blank casting; the heat treatment includes Method 1 or Method 2; Method 1 includes performing normalizing treatment and first tempering treatment in sequence, and the holding temperature of the normalizing treatment is 30 - 80°C above the AC3 temperature; Method 2 includes performing subcritical normalizing treatment and second tempering treatment in sequence, and the holding temperature of the subcritical normalizing treatment is greater than the AC1 temperature and less than the AC3 temperature. The present invention can significantly improve the low-temperature impact toughness of the cast steel component cast by an intermediate frequency induction furnace by optimizing the chemical composition of the cast steel component, simultaneously improving the purity of molten steel by the deoxidizer and the refining agent, and optimizing the method and control parameters of heat treatment, can meet the use safety of bridge construction under extremely cold climate conditions, and reduce the production cost of the cast steel component, and reduce resource consumption and environmental pollution. Specific embodiments

[0031] The present invention provides a method for preparing a cast steel component, comprising the following steps:

[0032] Melting the raw materials for preparing the cast steel component to obtain a melt; the cast steel component comprises elements with the following mass percentages: C 0.17 - 0.2%, Si 0.2 - 0.44%, Mn 1.1 - 1.3%, P ≤ 0.025%, S ≤ 0.025%, Cr 0.15 - 0.3%, Ni 0.3 - 0.5%, Mo ≤ 0.17%, Cu 0.15 - 0.45%, and the balance is Fe;

[0033] Placing a deoxidizer and a refining agent at the bottom of a ladle, pouring the melt into the ladle for deoxidation refining to obtain a refined melt;

[0034] Pouring the refined melt to obtain a blank casting;

[0035] Performing heat treatment on the blank casting; the heat treatment includes Method 1 or Method 2; Method 1 includes performing normalizing treatment and first tempering treatment in sequence, and the holding temperature of the normalizing treatment is 30 - 80°C above the AC3 temperature; Method 2 includes performing subcritical normalizing treatment and second tempering treatment in sequence, and the holding temperature of the subcritical normalizing treatment is greater than the AC1 temperature and less than the AC3 temperature.

[0036] In the present invention, unless otherwise specified, all preparation raw materials / components are commercially available products well-known to those skilled in the art.

[0037] By optimizing the chemical composition of the cast steel parts, simultaneously improving the purity of the molten steel through deoxidizers and refining agents, and optimizing the methods and control parameters of heat treatment, the present invention can significantly improve the low-temperature impact toughness of the cast steel parts cast by medium-frequency induction furnaces, meet the use safety requirements of bridge construction under extremely cold climatic conditions, reduce the production cost of the cast steel parts, and reduce resource consumption and environmental pollution.

[0038] The present invention melts the preparation raw materials of the cast steel parts to obtain a melt. There are no special requirements for the preparation raw materials of the cast steel parts, and the types of preparation raw materials of the cast steel parts well-known to those skilled in the art can be used, such as one or more of scrap steel, steel chips, return materials, and master alloys (such as ferrosilicon master alloy and / or ferromanganese master alloy). The melting process is carried out in a medium-frequency induction furnace, and the tapping temperature of the melt is preferably 1640 - 1680 °C, more preferably 1660 - 1680 °C. The present invention has no special requirements for the specific implementation process of the melting.

[0039] The cast steel parts include the following elements by mass percentage: C 0.17 - 0.2%, Si 0.2 - 0.44%, Mn 1.1 - 1.3%, P ≤ 0.025%, S ≤ 0.025%, Cr 0.15 - 0.3%, Ni 0.3 - 0.5%, Mo ≤ 0.17%, Cu 0.15 - 0.45%, and the balance is Fe. In the present invention, compared with ZG20Mn in the Technical Conditions for Large Low-Alloy Cast Steel Parts JB / T 6402 - 2018, the Mn content of the cast steel parts is adjusted to 1.1 - 1.3%, and the C content is adjusted to 0.17 - 0.2%. By increasing the content of the Mn element and simultaneously reducing the content of the C element, the low-temperature impact toughness of the cast steel parts is improved. This is because the Mn element can improve the strength and hardness of the steel, and Mn has the functions of deoxidation and desulfurization, which can purify the molten iron. The increase of the C element will significantly deteriorate the plasticity and toughness of the steel. Therefore, by reducing the content of the C element and increasing the content of the Mn element in the present invention, the toughness of the product can be improved, the inclusions can be reduced, and the low-temperature impact toughness of the cast steel parts can be effectively improved.

[0040] After obtaining the melt, the present invention places the deoxidizer and the refining agent at the bottom of the ladle, and pours the melt into the ladle for deoxidation refining to obtain a refined melt.

[0041] In the present invention, the deoxidation refining is secondary refining outside the furnace. The deoxidizer and refining agent are placed at the bottom of the ladle, and the melt is poured into the ladle to obtain a refined melt. During the deoxidation refining process, the temperature of the melt (temperature in the ladle) is preferably not lower than 1620 °C. The deoxidizer is specifically a silicon-aluminum-barium-vanadium deoxidizer, and the mass ratio of the deoxidizer to the mass of the melt is preferably 2.5 - 3.5‰, more preferably 3.0 - 3.5‰. The residual aluminum content in the refined melt is preferably ≤0.05%. If the residual aluminum content in the refined melt is too high, rod-shaped crystal fractures will occur, reducing the size of the fracture edge, thereby deteriorating the plasticity, toughness, and low-temperature impact toughness of the cast steel parts. By controlling the residual aluminum content in the refined melt, the present invention avoids the occurrence of the above disadvantages.

[0042] In the present invention, the refining agent is rare earth silicon, and the mass ratio of the refining agent to the mass of the melt is preferably 1 - 2‰, more preferably 1.5 - 2‰. The present invention has no special requirements for the sources of the silicon-aluminum-barium-vanadium deoxidizer and rare earth silicon, and commercially available products can be used. By using the rare earth silicon as the refining agent for secondary refining outside the furnace, the present invention can effectively remove one or more harmful impurities that may be contained in the molten steel, such as sulfur, oxides, and non-metallic inclusions, so as to improve the purity of the steel and reduce defects.

[0043] In the present invention, before the pouring, it is preferably further included to let the refined melt stand still. The standing still is preferably carried out in the ladle, the standing still time is preferably ≥5 min, preferably 5 - 10 min, and the standing still temperature is not lower than 1600 °C.

[0044] The present invention pours the refined melt. The pouring is carried out by a bottom-pouring ladle, and the pouring temperature is not lower than 1580 °C. If the temperature is lower than 1580 °C during the pouring process, the fluidity of the melt will be low, resulting in defects such as incomplete filling, cold shut, slag inclusion, and porosity. After pouring, cooling, opening the box, and cutting the riser, a rough cast is obtained.

[0045] The present invention heat-treats the rough cast in an electric resistance furnace. The main methods include (1) normalizing treatment + first tempering treatment; (2) subcritical normalizing treatment + second tempering treatment; wherein, the holding temperature of the normalizing treatment is 30 - 80 °C above the AC3 temperature to obtain a uniform austenite structure; the holding temperature of the subcritical normalizing treatment is higher than the AC1 temperature and lower than the AC3 temperature to obtain a ferrite + austenite structure; the AC3 temperature is preferably 840 - 850 °C, and the AC1 temperature is preferably 720 - 740 °C. In the present invention, the heat treatment is charged at room temperature. The present invention uses the heat treatment method of normalizing treatment + high-temperature tempering treatment (or subcritical normalizing treatment + high-temperature tempering treatment) to replace the traditional quenching and tempering treatment, reducing the cost while ensuring the product performance of the cast steel parts.

[0046] In the present invention, the holding temperature of the normalizing treatment is preferably 60 - 70 °C above the AC3 temperature, and the holding temperature of the normalizing treatment in the embodiment is specifically 920 °C. The holding time of the normalizing treatment is determined by the thickness of the cast steel part. The calculation formula for the holding time of the normalizing treatment is: holding time of the normalizing treatment (min) = wall thickness of the cast steel part (mm) × temperature coefficient, where the temperature coefficient is 1.5 - 3.5 min / mm. In a specific embodiment of the present invention, the holding time of the normalizing treatment is preferably 4 - 5 h. The heating rate of the normalizing treatment preferably includes: heating from room temperature to the low-temperature section temperature for holding, and after the holding ends, heating from the low-temperature section temperature to the high-temperature section temperature for holding. The low-temperature section temperature is preferably < AC1 temperature. In a specific embodiment of the present invention, the low-temperature section temperature is specifically 650 °C, and the holding time of the low-temperature section temperature is preferably 1 - 2 h. The present invention has no special requirements for the heating rate from room temperature to the low-temperature section temperature, and it can be set according to the power of the resistance furnace. The heating rate from the low-temperature section temperature to the high-temperature section temperature is preferably > 200 °C / h. In the present invention, the high-temperature section temperature is the holding temperature of the normalizing treatment. During the normalizing treatment process of the present invention, by setting the holding at the low-temperature section temperature, the temperature balance inside and outside the casting can be ensured. The present invention controls the heating rate from the low-temperature section temperature to the high-temperature section temperature to be preferably > 200 °C / h, which can shorten the residence time in the range from the low-temperature section temperature to the high-temperature section temperature (AC1 temperature to AC3 temperature range), thereby reducing the quantity and severity of the inheritance of acicular austenite caused by long-time residence. At the same time, rapid heating can increase the superheat degree, thereby improving the activity of atoms inside the metal, contributing to improving the original structure and compositional uniformity, and having a significant effect on optimizing the metal properties.

[0047] In the present invention, the cooling method after the normalizing treatment is preferably water mist cooling. The water mist used for water mist cooling is generated by the equipment for water mist cooling. The water mist preferably fully covers the cast steel part that has undergone the normalizing treatment to avoid the situation where uneven cooling rates result in different performances at different positions of the product.

[0048] In the present invention, the interval time between the first tempering treatment and the normalizing treatment is preferably ≤ 12 h. The holding temperature of the first tempering treatment is preferably 600 - 640 °C, and specifically 620 °C in the embodiment. The present invention selects the holding temperature of the first tempering treatment to be preferably 600 - 640 °C to avoid temper brittleness. At the same time, if the temperature of the first tempering treatment is too low, it will cause the product to have too high strength and hardness and too low plasticity and toughness. The holding time of the first tempering treatment is preferably 6 - 8 h, and specifically 7 h in the embodiment. In the present invention, the cooling method of the first tempering treatment is air cooling.

[0049] In the present invention, the holding temperature of the subcritical normalizing treatment is greater than the AC1 temperature and less than the AC3 temperature, specifically 830 °C in the embodiment. The holding time of the subcritical normalizing treatment is determined by the thickness of the steel casting. The calculation formula for the holding time of the normalizing treatment is: holding time of the subcritical normalizing treatment (min) = wall thickness of the steel casting (mm) × temperature coefficient, where the temperature coefficient is 1.5 - 3.5 min / mm. In a specific embodiment of the present invention, the holding time of the subcritical normalizing treatment is preferably 4 - 5 h. The heating rate of the subcritical normalizing treatment preferably includes: heating from room temperature to the low-temperature section temperature for holding, and after the holding ends, heating from the low-temperature section temperature to the high-temperature section temperature for holding. The low-temperature section temperature is preferably < AC1 temperature. In a specific embodiment of the present invention, the low-temperature section temperature is specifically 650 °C, and the holding time of the low-temperature section temperature is preferably 1 - 2 h. The present invention has no special requirements for the heating rate from room temperature to the low-temperature section temperature, and it can be set according to the power of the resistance furnace. The heating rate from the low-temperature section temperature to the high-temperature section temperature is preferably > 200 °C / h. In the present invention, the holding time at the high-temperature section temperature is the holding time of the subcritical normalizing treatment. During the subcritical normalizing treatment of the present invention, by holding at the low-temperature section temperature, the temperature balance inside and outside the casting can be ensured. The present invention controls the heating rate from the low-temperature section temperature to the high-temperature section temperature to be preferably > 200 °C / h, which can shorten the residence time in the range from the low-temperature section temperature to the high-temperature section temperature (AC1 temperature to AC3 temperature), thereby reducing the quantity and severity of the inheritance of acicular austenite caused by long-term residence. At the same time, rapid heating can increase the superheat degree, thereby improving the activity of atoms inside the metal, contributing to improving the original structure and compositional uniformity, and having a significant effect on optimizing the metal properties.

[0050] In the present invention, the cooling method after the subcritical normalizing treatment is preferably water mist cooling. The water mist used for the water mist cooling is generated by the equipment for water mist cooling. The water mist preferably fully covers the steel casting after the subcritical normalizing treatment to avoid the situation where different properties at different positions of the product are caused by uneven cooling rates.

[0051] In the present invention, the interval time between the second tempering treatment and the subcritical normalizing treatment is preferably ≤ 12 h. The holding temperature of the second tempering treatment is preferably 600 - 640 °C, specifically 620 °C in the embodiment. The present invention selects the holding temperature of the second tempering treatment to be preferably 600 - 640 °C to avoid temper brittleness. At the same time, if the temperature of the first tempering treatment is too low, the strength and hardness of the product will be too high, and the plasticity and toughness will be too low. The holding time of the first tempering treatment is preferably 6 - 8 h, specifically 7 h in the embodiment. In the present invention, the cooling method of the second tempering treatment is air cooling.

[0052] The present invention strictly controls the composition ratio of steel castings and the purity of the melt, and can use the heat treatment methods of normalizing + high-temperature tempering combined with subcritical normalizing + high-temperature tempering to replace the traditional quenching and tempering treatment, reducing the cost while ensuring the product performance.

[0053] In the present invention, the -40°C impact toughness AK of the steel casting V2 has an average value ≥ 22 J and a single value ≥ 15 J.

[0054] The present invention provides the application of the steel casting described in the above technical solution in bridge engineering. In the present invention, the steel casting is specifically applied to bridge engineering in extreme climate regions such as frozen soil and alpine regions.

[0055] In order to further illustrate the present invention, the technical solutions provided by the present invention will be described in detail below in conjunction with embodiments, but they cannot be understood as limiting the protection scope of the present invention.

[0056] The mass contents of each element in the steel castings in the following Examples 1-2 and Comparative Example 1 are shown in Table 1, and the remaining is Fe element.

[0057] Table 1 Composition and mass percentage content (%) of each example and comparative example

[0058]

[0059] Example 1

[0060] The production process of the steel casting provided in this example: melting → deoxidation refining → standing → pouring → cooling → unpacking → cutting risers → heat treatment → performance testing.

[0061] According to the mass percentage content of each element in the steel casting of Example 1 in Table 1, the mass raw materials of the steel casting are placed in an intermediate frequency induction furnace for melting to obtain a melt.

[0062] The obtained melt is subjected to secondary refining outside the furnace. Silicon-aluminum-barium-vanadium deoxidizer and rare earth silicon are placed at the bottom of the ladle. The mass of the silicon-aluminum-barium-vanadium deoxidizer is 3% of the mass of the melt, and the mass of the rare earth silicon is 1.5‰ of the mass of the melt. The melt is placed in the ladle for deoxidation refining. The tapping temperature of the melt is 1650°C, and the temperature in the ladle after tapping is not lower than 1620°C. The standing time in the ladle is ≥ 5 min, the pouring initial temperature is 1600°C, and the melt temperature during pouring is not lower than 1580°C. The pouring is carried out by a bottom-pouring ladle, followed by cooling, unpacking, and cutting risers to obtain an ingot.

[0063] The obtained ingot is subjected to heat treatment. The heat treatment method includes Method 1:

[0064] Method 1 is normalizing + high-temperature tempering heat treatment: After loading the ingot into the furnace at room temperature, it is heated to 650°C and held for 2 hours, then heated to 920°C at a heating rate of 200°C / h and held for 5 hours, and then water-spray cooled to room temperature. Immediately temper within 12 hours. The tempering temperature is selected as 620°C and held for 7 hours, and then air-cooled. Take the in-furnace test bars for testing. The testing includes one tensile test bar and two impact test bars.

[0065] Comparative Example 1

[0066] It is basically the same as the preparation method of Example 1, except that: the mass percentage content of each element in the cast steel part conforms to ZG20Mn in the technical conditions of JB / T 6402 low-alloy cast steel parts. The composition is shown in Table 1. The normalizing + high-temperature tempering heat treatment method in Example 1 is adopted, and the specific parameters are the same as those in Example 1.

[0067] Example 2

[0068] It is basically the same as the preparation method of Example 1, except that: the obtained ingot is subjected to heat treatment. The heat treatment method includes Method 2:

[0069] Method 2 is subcritical normalizing + high-temperature tempering treatment: After loading the ingot into the furnace at room temperature, it is heated to 650°C and held for 2 hours, then heated to 830°C at a heating rate of 200°C / h and held for 5 hours, and then water-spray cooled to room temperature. Immediately temper within 12 hours. The tempering temperature is selected as 620°C and held for 7 hours, and then air-cooled. Take the in-furnace test bars for testing. The testing includes one tensile test bar and two impact test bars.

[0070] Test Example

[0071] The tensile test bars and impact test bars prepared in Example 1 and 2 and Comparative Example 1 are subjected to performance testing. Among them, the tensile test is carried out in accordance with GB / T 228.1-2021 Metallic materials - Tensile testing - Part 1: Method of test at room temperature; GB / T 229-2020 Metallic materials - Charpy pendulum impact test method.

[0072] Table 2 Mechanical property results of examples and comparative examples

[0073]

[0074] It can be seen from the test results of Example 1 and Comparative Example 1 in Table 2 that the larger the ratio of Mn content to C content, the higher the low-temperature impact energy. Therefore, in the smelting of the present invention, the C content needs to be reduced and the Mn content increased to ensure the qualified product performance.

[0075] It can be seen from the test results of Example 2 that subcritical normalizing + tempering can also meet the customer's requirements. This is because the undissolved ferrite can serve as the crystallization core during normalizing cooling, which is beneficial to reducing or inhibiting the precipitation of ferrite along the austenite grain boundaries, and can refine the grain size and improve the impact performance of the material.

[0076] As can be seen from the above examples, the present invention can further optimize the low-temperature impact performance by optimizing the chemical composition ratio of the cast steel, controlling the deoxidation, refining and tapping temperature during the melting process, and cooperating with an appropriate heat treatment process. It replaces the traditional melting method of electric arc furnace + refining, and the heat treatment method is also replaced by normalizing + tempering instead of quenching and tempering, reducing the manufacturing cost.

[0077] Although the above embodiments have made a detailed description of the present invention, they are only a part of the embodiments of the present invention, rather than all embodiments. Other embodiments can be obtained according to these embodiments without creative efforts, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for preparing a steel casting, characterized in that: The following steps are involved: The raw materials for preparing the steel casting are smelted to obtain a melt; the steel casting comprises the following elements in percentage by weight: C 0.17-0.2%, Si 0.2-0.44%, Mn 1.1-1.3%, P≤0.025%, S≤0.025%, Cr 0.15-0.3%, Ni 0.3-0.5%, Mo≤0.17%, Cu 0.15-0.45%, and the balance Fe; Placing a deoxidizer and a refining agent at the bottom of a ladle, and pouring the melt into the ladle for deoxidation and refining to obtain a refined melt; pouring the refined melt to obtain a blank casting; The blank casting is heat treated; the heat treatment includes method 1 or method 2; the method 1 includes normalizing treatment and first tempering treatment in sequence, the holding temperature of the normalizing treatment is 30-80°C above the AC3 temperature, the heating procedure of the normalizing treatment includes: heating from room temperature to the low temperature section temperature for heating, and heating from the low temperature section temperature to the high temperature section temperature for heating after the heating is completed, the temperature of the low temperature stage is less than the AC1 temperature, the high temperature section temperature is the holding temperature of the normalizing treatment, and the heating rate from the low temperature section temperature to the high temperature section temperature is greater than 200°C / h; The second method includes sequentially performing a sub-temperature normalizing treatment and a second tempering treatment, wherein the holding temperature of the sub-temperature normalizing treatment is greater than the AC1 temperature and less than the AC3 temperature, and the heating procedure of the sub-temperature normalizing treatment includes: heating from room temperature to a low temperature section temperature for insulation, and heating from the low temperature section temperature to a high temperature section temperature for insulation after the insulation is completed, wherein the temperature of the low temperature stage is less than the AC1 temperature, the high temperature section temperature is the holding temperature of the sub-temperature normalizing treatment, and the heating rate from the low temperature section temperature to the high temperature section temperature is greater than 200°C / h.

2. The preparation method according to claim 1, characterized in that: The smelting is carried out in a medium frequency induction furnace; the outlet temperature of the melt is 1640-1680°C.

3. The preparation method according to claim 1, characterized in that: The deoxidation refining is refining outside the furnace; The deoxidizer is a silicon aluminum barium vanadium deoxidizer, and the mass of the deoxidizer accounts for 2.5-3.5‰ of the mass of the melt; the amount of residual aluminum in the refined melt is ≤0.05wt%; The refining agent is a rare earth silicon alloy, and the mass of the refining agent accounts for 1-2‰ of the mass of the melt.

4. The preparation method according to claim 1 or 3, characterized in that: The deoxidation refining is carried out in a ladle, and the temperature in the ladle is not less than 1620°C; After obtaining the refined melt and before the pouring, the method further comprises: allowing the refined melt to stand still, wherein the standing still is performed in the ladle, the standing still time is ≥5 minutes, and the standing still temperature is not less than 1600° C.; The pouring temperature is not less than 1580°C; and the pouring is performed by ladle pouring.

5. The preparation method according to claim 1, characterized in that: The AC3 temperature is 840-850°C, and the AC1 temperature is 720-740°C.

6. The preparation method according to claim 1 or 5, characterized in that: The insulation time of the low temperature section temperature is 1 to 2 hours; The holding time of the high temperature section temperature is 2 to 5 hours.

7. The preparation method according to claim 1, characterized in that: The holding temperatures of the first tempering treatment and the second tempering treatment are independently 600-640° C.; The holding time of the first tempering treatment and the second tempering treatment is independently 6 to 8 hours; The cooling method after the normalizing treatment or the sub-temperature normalizing treatment is water mist cooling; the interval time between the first tempering treatment and the normalizing treatment is ≤12h; the interval time between the second tempering treatment and the sub-temperature normalizing treatment is ≤12h; the cooling method of the first tempering treatment and the second tempering treatment is air cooling.

8. The steel casting obtained by the preparation method according to any one of claims 1 to 7.

9. The steel casting according to claim 8, characterized in that -40℃ impact toughness AK of the steel casting V2 The mean value is ≥22J and the single value is ≥15J.

10. Use of the steel casting according to claim 8 or 9 in bridge engineering.

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