Steel casting and preparation method and application thereof

Abstract

The invention belongs to the technical field of metallurgical materials, and particularly relates to a steel casting and a preparation method and application thereof. The preparation method of the steel casting comprises the following steps that preparation raw materials of the steel casting are smelted, and a melt is obtained; the melt, a deoxidizing agent and a refining agent are mixed for deoxidizing refining and pouring, and a blank casting is obtained; and the blank casting is subjected to heat treatment, and the steel casting is obtained. By optimizing the chemical components of the steel casting, improving the purity of the molten steel through the deoxidizing agent and the refining agent and optimizing the heat treatment method and control parameters, the low-temperature impact toughness of the steel casting cast by the medium-frequency induction furnace can be obviously improved, and the use safety of bridge construction under the extremely cold weather condition can be met; and the production cost of the steel casting is reduced, and resource consumption and environmental pollution are reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgical materials, and in particular relates to a steel casting and a preparation method and application thereof. Background Art

[0002] At present, bridge construction has become a trend across all continents of the world, and its footprint is steadily extending to extreme climate areas such as frozen soil and high cold (-60℃~60℃). This trend has prompted bridge engineering to put forward more stringent requirements on the cold resistance of materials. Among the many cold resistance performance evaluation indicators, low-temperature impact toughness occupies a pivotal position, which is directly related to the structural safety and durability of bridges in extreme low temperature environments. The optimization of this performance is deeply affected by multiple factors such as chemical composition regulation, fine design of material microstructure, and improvement of molten steel purity.

[0003] In order to ensure that the molten steel reaches a highly pure state such as being free of pores and slag, traditionally, companies have mostly used electric arc furnaces combined with complex refining processes to remove impurities with sophisticated means to obtain steel castings with better 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 change is the reality that the electric arc furnace has a large loss of electrodes and metal raw materials during the production process, accompanied by a large amount of harmful gases and particulate matter emissions. However, since medium frequency induction furnace technology can only remove some impurities, it cannot achieve the same refined impurity removal as electric arc furnace technology. The low-temperature impact toughness of the resulting steel castings is poor, which cannot meet the safety requirements of bridge construction in extremely cold climate conditions. Summary of the invention

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

[0006] In order to achieve the above object, the present invention provides the following technical solutions: The present invention provides a method for preparing a steel casting, comprising the following steps: 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%, Ni0.3-0.5%, 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 one or method two; the method one includes performing normalizing treatment and first tempering treatment in sequence, and the holding temperature of the normalizing treatment is 30~80℃ above the AC3 temperature; the method two includes performing sub-temperature normalizing treatment and second tempering treatment in sequence, and the holding temperature of the sub-temperature normalizing treatment is greater than the AC1 temperature and less than the AC3 temperature.

[0007] Preferably, the smelting is carried out in a medium frequency induction furnace; the furnace outlet temperature of the melt is 1640-1680°C.

[0008] Preferably, 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.

[0009] Preferably, 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.

[0010] Preferably, the AC3 temperature is 840-850°C, and the AC1 temperature is 720-740°C.

[0011] Preferably, the temperature raising procedure of the normalizing treatment or the sub-temperature normalizing treatment comprises: heating from room temperature to the low temperature section temperature and then keeping the temperature, and after the temperature is kept, heating from the low temperature section temperature to the high temperature section temperature and then keeping the temperature; The insulation time of the low temperature section temperature is 1 to 2 hours; The heating rate from the low temperature section to the high temperature section is greater than 200°C / h; The holding time of the high temperature section temperature is 2 to 5 hours.

[0012] Preferably, 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.

[0013] The present invention provides a steel casting prepared by the preparation method described in the above technical solution.

[0014] Preferably, the -40°C impact toughness AK of the steel casting is V2 The mean value is ≥22J and the single value is ≥15J.

[0015] The present invention provides the application of the steel casting described in the above technical solution in bridge engineering.

[0016] The invention provides a method for preparing a steel casting, comprising the following steps: smelting raw materials for preparing the steel casting 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%, the balance is Fe; placing a deoxidizer and a refining agent at the bottom of the ladle, pouring the melt into the ladle for deoxidation and refining to obtain a refined melt; pouring the refined melt to obtain a blank casting; heat treating the blank casting; the heat treatment includes method one or method two; the method one includes sequentially performing normalizing treatment and a first tempering treatment, and the insulation temperature of the normalizing treatment is 30~80°C above the AC3 temperature; the method two includes sequentially performing a sub-temperature normalizing treatment and a second tempering treatment, and the insulation temperature of the sub-temperature 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 steel castings cast by a medium-frequency induction furnace by optimizing the chemical composition of steel castings, and at the same time improve the purity of molten steel by using deoxidizers and refining agents, and optimize the method and control parameters of heat treatment, so as to meet the safety of bridge construction under extremely cold climate conditions, reduce the production cost of steel castings, and reduce resource consumption and environmental pollution. DETAILED DESCRIPTION

[0017] The present invention provides a method for preparing a steel casting, comprising the following steps: 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%, Ni0.3-0.5%, 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 one or method two; the method one includes performing normalizing treatment and first tempering treatment in sequence, and the holding temperature of the normalizing treatment is 30~80℃ above the AC3 temperature; the method two includes performing sub-temperature normalizing treatment and second tempering treatment in sequence, and the holding temperature of the sub-temperature normalizing treatment is greater than the AC1 temperature and less than the AC3 temperature.

[0018] In the present invention, unless otherwise specified, all preparation raw materials / components are commercially available products well known to those skilled in the art.

[0019] The present invention can significantly improve the low-temperature impact toughness of steel castings cast in a medium-frequency induction furnace by optimizing the chemical composition of steel castings, improving the purity of molten steel by using deoxidizers and refining agents, and optimizing the heat treatment method and control parameters, thereby meeting the safety requirements of bridge construction under extremely cold climate conditions, reducing the production cost of steel castings, and reducing resource consumption and environmental pollution.

[0020] The present invention melts the raw materials for preparing steel castings to obtain a melt. There are no special requirements for the raw materials for preparing steel castings. The types of raw materials for preparing steel castings well known to those skilled in the art can be used, such as: one or more of scrap steel, steel scraps, recycled materials, and master alloys (such as ferrosilicon master alloys and / or ferromanganese master alloys). The smelting process is carried out in a medium frequency induction furnace, and the furnace 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 smelting.

[0021] The steel casting includes the following elements in percentage by mass: 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. In the present invention, compared with ZG20Mn in the technical conditions for large low-alloy steel castings in JB / T 6402-2018, the Mn content of the steel casting is adjusted to 1.1-1.3%, and the C content is adjusted to 0.17-0.2%, which is to improve the low-temperature impact toughness of the steel casting by increasing the content of the Mn element and reducing the content of the C element. This is because the Mn element can improve the strength and hardness of steel, and Mn has the effects of deoxidation and desulfurization, and can purify molten iron, while the increase of the C element will cause the plasticity and toughness of the steel to deteriorate significantly. Therefore, the present invention reduces the content of the C element and increases the content of the Mn element, which can improve the toughness of the product and reduce inclusions, so as to effectively improve the low-temperature impact toughness of the steel casting.

[0022] After obtaining the melt, the present invention places a deoxidizer and a refining agent at the bottom of a ladle, pours the melt into the ladle for deoxidation and refining, and obtains a refined melt.

[0023] In the present invention, the deoxidation refining is an out-of-furnace refining, which is to place the deoxidizer and the refining agent at the bottom of the ladle, and pour the melt into the ladle to obtain a refined melt. The melt temperature (temperature in the ladle) during the deoxidation refining process is preferably not less than 1620°C. The deoxidizer is specifically a silicon aluminum barium vanadium deoxidizer, and the mass of the deoxidizer accounts for 2.5~3.5‰ of the mass of the melt, and more preferably 3.0~3.5‰. The amount of residual aluminum in the refined melt is preferably ≤0.05%. If the residual aluminum content in the refined melt is too high, a rod-shaped crystal fracture will be produced, which will reduce the size of the fracture edge, thereby deteriorating the plasticity, toughness and low-temperature impact toughness of the steel casting. The present invention avoids the above-mentioned disadvantages by controlling the content of residual aluminum in the refined melt.

[0024] In the present invention, the refining agent is rare earth silicon, and the mass of the refining agent accounts for preferably 1-2‰ of the mass of the melt, and more preferably 1.5-2‰. The present invention has no special requirements on the sources of the silicon, aluminum, barium and vanadium deoxidizer and rare earth silicon, and commercially available products can be used. The present invention uses the rare earth silicon as a refining agent for off-furnace refining, which can effectively remove harmful impurities that may be contained in molten steel, such as one or more of sulfur, oxides and non-metallic inclusions, to improve the purity of steel and reduce defects.

[0025] In the present invention, the refined melt is preferably placed in a standing position before the pouring. The standing position is preferably carried out in the ladle, the standing time is preferably ≥5 min, preferably 5-10 min, and the standing temperature is not less than 1600°C.

[0026] The present invention pours the refined melt, and the pouring adopts ladle pouring, and the pouring temperature is not less 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 air holes. After pouring, cooling, unpacking, and cutting the riser, a blank casting is obtained.

[0027] The present invention heat treats the blank casting in a resistance furnace, and the main methods include (1) normalizing treatment + first tempering treatment; (2) sub-temperature 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 sub-temperature normalizing treatment is greater than the AC1 temperature and less 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 adopts a heat treatment method of normalizing treatment + high temperature tempering treatment (or sub-temperature normalizing treatment + high temperature tempering treatment) to replace the traditional quenching and tempering treatment, thereby reducing costs while ensuring the performance of the steel casting product.

[0028] 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 steel casting. The calculation formula for the holding time of the normalizing treatment is: holding time of normalizing treatment (min) = wall thickness of the steel casting (mm) × temperature coefficient, wherein the temperature coefficient is 1.5-3.5min / mm. In a specific embodiment of the present invention, the holding time of the normalizing treatment is preferably 4-5h. The temperature rising sequence of the normalizing treatment preferably includes: heating from room temperature to the low temperature section temperature for insulation, and heating from the low temperature section temperature to the high temperature section temperature for insulation after the insulation is completed. 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 1-2h. 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 insulation temperature of the normalizing treatment. During the normalizing treatment, the present invention can ensure the temperature balance inside and outside the casting by setting the insulation under the low temperature section temperature condition. 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 stay in the range of the low temperature section temperature to the high temperature section temperature (AC1 temperature to AC3 temperature range), thereby reducing the number and severity of needle-shaped austenite caused by long-term stay. At the same time, rapid heating can increase the superheat, thereby increasing the activity of atoms inside the metal, which helps to improve the original organization and composition uniformity, and has a significant effect on the optimization of metal properties.

[0029] In the present invention, the cooling method after the normalizing treatment is preferably water mist cooling. The water mist used in the water mist cooling is generated by a water mist cooling device. The water mist preferably fully covers the normalizing steel casting to avoid the situation where the performance of different positions of the product is different due to uneven cooling rate.

[0030] In the present invention, the interval time between the first tempering treatment and the normalizing treatment is preferably ≤12h. The insulation temperature of the first tempering treatment is preferably 600~640℃, and specifically 620℃ in the embodiment. The present invention selects the insulation temperature of the first tempering treatment to be preferably 600~640℃, which can avoid temper brittleness. At the same time, if the temperature of the first tempering treatment is too low, the product strength and hardness will be too high, and the plasticity and toughness will be too low. The insulation time of the first tempering treatment is preferably 6~8h, and specifically 7h in the embodiment. In the present invention, the cooling method of the first tempering treatment is air cooling.

[0031] In the present invention, the holding temperature of the sub-temperature normalizing treatment is greater than the AC1 temperature and less than the AC3 temperature, and is specifically 830°C in the embodiment. The holding time of the sub-temperature 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 sub-temperature normalizing treatment (min) = wall thickness of the steel casting (mm) × temperature coefficient, where the temperature coefficient is 1.5~3.5min / mm. In a specific embodiment of the present invention, the holding time of the sub-temperature normalizing treatment is preferably 4~5h. The temperature rising sequence of the sub-temperature normalizing treatment preferably includes: heating from room temperature to the low temperature section temperature for insulation, and heating from the low temperature section temperature to the high temperature section temperature for insulation after the insulation is completed. 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 1~2h. 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 of the high temperature section temperature is the holding time of the sub-temperature normalizing treatment. During the sub-temperature normalizing treatment, the present invention can ensure the temperature balance inside and outside the casting by keeping warm under the low temperature section temperature condition. 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 stay in the range of the low temperature section temperature to the high temperature section temperature (AC1 temperature to AC3 temperature), thereby reducing the number and severity of needle-shaped austenite caused by long-term stay, and rapid heating can increase the superheat, thereby increasing the activity of atoms inside the metal, which helps to improve the original organization and composition uniformity, and has a significant effect on the optimization of metal properties.

[0032] In the present invention, the cooling method after the sub-temperature normalizing treatment is preferably water mist cooling. The water mist used in the water mist cooling is generated by a water mist cooling device. The water mist preferably fully covers the steel castings after the sub-temperature normalizing treatment to avoid the situation where the performance of different positions of the product is different due to uneven cooling rate.

[0033] In the present invention, the interval time between the second tempering treatment and the sub-temperature normalizing treatment is preferably ≤12h. The insulation temperature of the second tempering treatment is preferably 600~640℃, and specifically 620℃ in the embodiment. The insulation temperature of the second tempering treatment selected by the present invention is preferably 600~640℃, which can avoid temper brittleness. At the same time, if the temperature of the first tempering treatment is too low, the product strength and hardness will be too high, and the plasticity and toughness will be too low. The insulation time of the first tempering treatment is preferably 6~8h, and specifically 7h in the embodiment. In the present invention, the cooling method of the second tempering treatment is air cooling.

[0034] The present invention strictly controls the proportion of the components of the steel casting and the purity of the melt, and can replace the traditional quenching and tempering treatment with a heat treatment method of normalizing + high-temperature tempering combined with sub-temperature normalizing + high-temperature tempering, thereby reducing costs while ensuring product performance.

[0035] In the present invention, the -40°C impact toughness AK of the steel casting is V2 The mean value is ≥22J and the single value is ≥15J.

[0036] 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 areas such as frozen soil and high cold.

[0037] In order to further illustrate the present invention, the technical solutions provided by the present invention are described in detail below in conjunction with embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0038] The mass contents of the elements in the steel castings in the following Examples 1-2 and Comparative Example 1 are shown in Table 1, and the remainder is Fe element.

[0039] Table 1 Composition and mass percentage of each embodiment and comparative example (%)

[0040] Example 1 The production process of the steel casting provided in this embodiment is as follows: smelting → deoxidation refining → standing → pouring → cooling → unpacking → cutting riser → heat treatment → performance testing.

[0041] According to the mass percentage of each element in the steel casting of Example 1 in Table 1, the mass raw material of the steel casting is placed in a medium frequency induction furnace for smelting to obtain a melt.

[0042] The obtained melt is refined outside the furnace, and silicon, aluminum, barium, vanadium deoxidizer and rare earth silicon are placed at the bottom of the ladle, where the mass of silicon, aluminum, barium, vanadium deoxidizer is 3% of the mass of the melt, and the mass of rare earth silicon is 1.5‰ of the mass of the melt. The melt is placed in the ladle for deoxidation and refining, the temperature of the melt out of the furnace is 1650℃, the temperature in the ladle after the melt is out of the furnace is not less than 1620℃, the static time in the ladle is ≥5min, the first pouring temperature is 1600℃, and the melt temperature during the pouring process is not less than 1580℃. The ladle is used for pouring, cooling, unpacking, cutting the riser, and obtaining the ingot.

[0043] The obtained ingot is heat treated, and the heat treatment method includes method 1: Method 1 is normalizing + high temperature tempering heat treatment: after the ingot is loaded into the furnace at room temperature, the temperature is raised to 650℃ and kept for 2h, then the temperature is raised to 920℃ at a heating rate of 200℃ / h and kept for 5h, then cooled to room temperature with water mist, and tempered immediately within 12 hours. The tempering temperature is selected to be 620℃ and kept for 7 hours, then air-cooled, and the test bars with the furnace are taken for testing, including one tensile test bar and two impact test bars.

[0044] Comparative Example 1 The preparation method is basically the same as that in Example 1, except that the mass percentage of each element in the steel casting conforms to the technical conditions of JB / T 6402 low alloy steel castings ZG20Mn, the composition is shown in Table 1, and the heat treatment method of normalizing + high temperature tempering in Example 1 is adopted, and the specific parameters are the same as in Example 1.

[0045] Example 2 The preparation method is basically the same as that of Example 1, except that the obtained ingot is subjected to heat treatment, and the heat treatment method includes method 2: Method 2 is sub-temperature normalizing + high-temperature tempering treatment: after the ingot is loaded into the furnace at room temperature, the temperature is raised to 650℃ and kept for 2h, then the temperature is raised to 830℃ at a heating rate of 200℃ / h and kept for 5h, then cooled to room temperature with water mist, and tempered immediately within 12 hours. The tempering temperature is selected to be 620℃, kept for 7 hours, and then air-cooled. The test bars with the furnace are taken for testing, including one tensile test bar and two impact test bars.

[0046] Test Case The tensile test bars and impact test bars prepared in Examples 1 and 2 and Comparative Example 1 were subjected to performance testing, wherein the tensile test was carried out in accordance with GB / T 228.1-2021 Tensile test of metallic materials Part 1: Room temperature test method; GB / T 229-2020 Charpy pendulum impact test method for metallic materials.

[0047] Table 2 Mechanical properties of the examples and comparative examples

[0048] It can be seen from the test results of Example 1 and Comparative Example 1 in Table 2 that the greater the ratio of Mn content to C content, the higher the low-temperature impact energy. Therefore, in the present invention, the C content needs to be reduced and the Mn content needs to be increased during smelting to ensure that the product performance is qualified.

[0049] It can be seen from the test results of Example 2 that sub-temperature normalizing + tempering can also meet customer needs. This is because the undissolved ferrite can be used as the crystallization core during normalizing cooling, which is beneficial to reduce or inhibit the precipitation of ferrite along the austenite grain boundaries, and can refine the grain size and improve the impact performance of the material.

[0050] It can be seen from the above embodiments that the present invention can further optimize the low-temperature impact performance by optimizing the chemical composition ratio of cast steel, controlling the deoxidation, refining and furnace temperature during the smelting process, and coordinating appropriate heat treatment processes, thereby replacing the traditional arc furnace + refining smelting method, and the heat treatment method is also replaced by normalizing + tempering instead of tempering treatment, thereby reducing the manufacturing cost.

[0051] Although the above embodiment describes the present invention in detail, it is only a part of the embodiments of the present invention, not all of the embodiments. Other embodiments can be obtained based on this embodiment without creativity, 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 one or method two; the method one includes performing normalizing treatment and first tempering treatment in sequence, and the holding temperature of the normalizing treatment is 30~80℃ above the AC3 temperature; the method two includes performing sub-temperature normalizing treatment and second tempering treatment in sequence, and the holding temperature of the sub-temperature normalizing treatment is greater than the AC1 temperature and less than the AC3 temperature.

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 heating procedure of the normalizing treatment or the sub-temperature normalizing treatment includes: heating from room temperature to the low temperature section temperature and keeping the temperature, and after the temperature is kept, heating from the low temperature section temperature to the high temperature section temperature and keeping the temperature; The insulation time of the low temperature section temperature is 1 to 2 hours; The heating rate from the low temperature section to the high temperature section is greater than 200°C / h; 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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