Low-temperature impact resistant cast steel

Abstract

The low-temperature impact resistant cast steel is prepared from the following components in percentage by weight: 0.15 to 0.20 percent of C, 0.22 to 0.39 percent of Si, 0.88 to 1.35 percent of Mn, 0.35 to 0.55 percent of Ni, 0.10 to 0.19 percent of Cr, 0.02 to 0.06 percent of Nb, 0.008 to 0.021 percent of Zr, less than or equal to 0.02 percent of P, less than or equal to 0.02 percent of S and the balance of Fe and inevitable impurities. The mass percentage contents [Cr], [Nb] and [Zr] of Cr, Nb and Zr meet the following content relational expression: 0.21 < = [Cr] + 9.57 [Zr] < = 0.33; 0.071 < = [Nb] + 4.22 [Zr] < = 0.12.

Description

Technical Field

[0001] The invention relates to a low-temperature impact-resistant cast steel, which has good room-temperature mechanical properties and low-temperature toughness matching, a low ductile-brittle transition temperature, and is suitable for use as a component of railway freight cars in high-latitude and cold regions. Background Art

[0002] Railway freight cars usually need to cope with heavy load conditions, and many of their parts are made of cast steel. Railway freight cars running in high latitudes and cold regions not only require parts to have excellent mechanical properties, but also have strict requirements on toughness under low temperature conditions. If the low-temperature toughness of cast steel is insufficient or the ductile-brittle transition temperature is high, it is easy to cause the parts of railway freight cars to break and fail in low-temperature conditions, affecting the normal operation of railway freight cars, and in serious cases, may cause loss of life and property.

[0003] Therefore, the present invention provides a cast steel having excellent room temperature mechanical properties and outstanding toughness under low temperature conditions. Summary of the invention

[0004] The present invention provides a low-temperature impact-resistant cast steel, which has excellent room-temperature mechanical properties and outstanding low-temperature toughness through the coordinated combination of components. The cast steel has a low ductile-brittle transition temperature and is suitable for use as a component of railway freight cars in high-latitude and cold regions.

[0005] The technical solution of the present invention is as follows.

[0006] The invention provides a low-temperature impact resistant cast steel. The chemical components of the low-temperature impact resistant cast steel are as follows by weight percentage: C: 0.15-0.20%, Si: 0.22-0.39%, Mn: 0.88-1.35%, Ni: 0.35-0.55%, Cr: 0.10-0.19%, Nb: 0.02-0.06%, Zr: 0.008-0.021%, P≤0.02%, S≤0.02%, and the rest are Fe and inevitable impurities; and the mass percentage contents [Cr], [Nb], [Zr] of Cr, Nb, and Zr satisfy the following content relationship formula: 0.21≤[Cr]+9.57[Zr]≤0.33; 0.071≤[Nb]+4.22[Zr]≤0.12.

[0007] In order to ensure excellent strength, traditional cast steel usually contains a high content of Cr. However, the inventors of the present invention have found that excessively high Cr content will lead to the generation of a large amount of chromium carbides (Cr7C3, Cr2C3) in the cast steel matrix. These chromium carbides are prone to become the starting point of fracture when subjected to low-temperature impact loads, resulting in poor low-temperature impact resistance of the cast steel, an imbalance between the room temperature mechanical properties and low-temperature impact resistance of the cast steel, and an inability to effectively match them.

[0008] Based on the above understanding, the present invention significantly reduces the content of Cr in cast steel to reduce chromium carbide in the cast steel matrix, thereby ensuring that the cast steel has excellent low-temperature toughness and outstanding ductile-brittle transition temperature. However, as recognized in the prior art, due to the solution strengthening effect of Cr, a relatively high content of Cr is crucial for ensuring the mechanical properties of cast steel. Simply reducing the Cr content will not effectively guarantee the room-temperature mechanical properties of the cast steel.

[0009] Therefore, while significantly reducing the Cr content, the present invention also adds Zr element. Zr is a carbide-forming element, which can refine the structure and grains of steel, improve the strength, and make up for the strength loss of cast steel caused by the reduction of Cr. However, if the content of Zr carbide is too high, it will also lead to the deterioration of the low-temperature impact resistance of the cast steel. The present invention also adds Nb. The inventors have found that an appropriate amount of Nb can not only improve the strength of cast steel, but also has a very prominent effect on improving the low-temperature impact resistance of cast steel. Therefore, the introduction of Nb in the present invention is mainly used to improve the low-temperature impact resistance of cast steel, and can also play a role in strengthening the matrix. However, excessive Nb will also cause the deterioration of the low-temperature impact resistance.

[0010] Therefore, while adding Cr, Zr, and Nb, the present invention also needs to limit the contents of Cr, Zr, and Nb. According to the repeated experiments of the inventors, it is confirmed that when the contents of Cr, Zr, and Nb satisfy 0.21 ≤ [Cr] + 9.57[Zr] ≤ 0.33 and 0.071 ≤ [Nb] + 4.22[Zr] ≤ 0.12, it can ensure that the cast steel matrix has good room-temperature mechanical properties and low-temperature impact resistance.

[0011] The anti-low-temperature impact cast steel with the above composition proposed by the present invention has the following room-temperature mechanical properties: tensile strength above 620 MPa, yield strength above 415 MPa, and elongation above 27.5%; its low-temperature impact resistance is: ductile-brittle transition temperature below -88 °C, and the impact toughness KV2 at -60 °C is above 65 J.

[0012] As a preferred technical solution, the anti-low-temperature cast steel of the present invention is preferably in a heat-treated state. Through heat treatment, the cast steel can have an excellent match of room-temperature properties and low-temperature properties.

[0013] The present invention also provides a preparation method of the foregoing anti-low-temperature impact cast steel, which is characterized by including the following steps: smelting to obtain molten steel with corresponding components, casting the molten steel into a mold to obtain cast steel, and heat-treating the cast steel.

[0014] There are no special limitations on the smelting and casting processes. Molten steel can be obtained by smelting hot metal from a blast furnace through a converter, or directly smelting molten steel in an EAF. The molten steel is refined, for example, through an AOD furnace / LF furnace + RH furnace / VD furnace, and finally molten steel with a composition meeting the requirements is obtained. The molten steel is cast into a mold, which can be a sand mold, or a lost foam mold or a sand-coated iron mold, etc.

[0015] Through the combination of composition and heat treatment process, the low-temperature impact-resistant cast steel of the present invention can exhibit particularly excellent room-temperature mechanical properties and low-temperature impact resistance characteristics.

[0016] As a non-limiting description, the heat treatment of the cast steel can sequentially include normalizing, quenching, and tempering. The cast steel obtained through the heat treatment process of the above three steps finally shows excellent room-temperature mechanical properties and outstanding low-temperature impact resistance characteristics.

[0017] Without being limited to the following, during the heat treatment process of the cast steel of the present invention, the normalizing temperature is 950 - 980 °C, the holding time is 0.5 - 2 h, the quenching temperature is 880 - 910 °C, the holding time is 0.5 - 2 h, the tempering temperature is 610 - 640 °C, and the holding time is 3 - 5 h. The quenching can be selected as oil quenching.

[0018] The present invention also provides the application of the aforementioned low-temperature impact-resistant cast steel or the low-temperature impact-resistant cast steel prepared by the aforementioned preparation method in railway wagon parts. Due to the excellent matching of the room-temperature mechanical properties and low-temperature impact resistance of the cast steel of the present invention, it performs outstandingly in the application of railway wagon parts in high-latitude and cold regions, can meet the operating conditions of low temperature and high load, has an outstanding service life, and is stable and reliable during the service process.

[0019] The technical effects achieved by the present invention are mainly manifested in: by reducing the Cr content in the cast steel to avoid the negative impact of chromium carbide on the low-temperature impact resistance of the cast steel, while adding Zr and Nb, which can make up for the loss of strength of the cast steel caused by the decrease in Cr content and ensure its appropriate mechanical properties. More importantly, a small amount of Nb has a positive effect on improving the low-temperature impact resistance of the cast steel. By adding Cr, Zr, and Nb and controlling the content of each of the above elements within a reasonable range, cast steel with relatively excellent room-temperature mechanical properties and low-temperature impact resistance can be obtained. When the cast steel of the present invention is used as railway wagon parts in high-latitude and cold regions with high loads, it exhibits an excellent service life and is stable and reliable during the service process. Detailed Embodiments

[0020] To enable those of ordinary skill in the art to fully understand the technical solutions and beneficial effects of the present invention, the following further explanations are made in combination with specific test examples.

[0021] A cast steel specimen with a diameter of 50 cm and a length of 120 cm was melted and cast. The specific composition analysis results are shown in Table 1, where formula a represents [Cr] + 9.57[Zr], formula b represents [Nb] + 4.22[Zr], and both P and S are controlled within 0.01% ± 0.001%. Heat treatment was carried out on the cast steel specimen as follows: first, normalizing treatment was carried out at 970 °C for 45 min, then air-cooled to room temperature, then heated to 890 °C and held for 1 h, followed by oil quenching to room temperature, and then heated to 625 °C and held for 3.5 h, and finally air-cooled to room temperature for performance testing. The test results are recorded in Table 2. Refer to GB / T228.1-2021 to test the mechanical properties of the cast steel at room temperature, including tensile strength, yield strength, and elongation. Refer to GB / T229-2020 to test the impact toughness KV2 of the cast steel at -60 °C and determine the ductile-brittle transition temperature (DBTT) of the cast steel based on a 50% brittle fracture rate (starting from -40 °C, with a measurement unit of every 2 °C).

[0022] Table 1: Composition of each cast steel, mass%, balance Fe.

[0023]

[0024] Table 2: Properties of each cast steel.

[0025]

[0026] For steel grades 7-13, the Cr content was adjusted. The Cr content of steel grades 8 and 11 is lower than the invention requirement range, the Cr content of steel grades 9 and 10 is higher than the invention requirement range, and the formula a of steel grades 8 and 10 is not within the invention requirement range. Steel grade 12 does not contain Cr, and although the Cr content of steel grade 13 is within the invention range, the formula a does not meet the invention requirement. It is not difficult to see that when the Cr content is too high, although it will bring an increase in strength, it will lead to the deterioration of the plasticity and low-temperature impact resistance of the cast steel; while when the Cr content is low or no Cr is added, the cast steel has good low-temperature impact resistance, but the room temperature strength is insufficient; especially for steel grade 13, although Cr is still within the invention range, due to the formula a being higher than the upper limit of the invention requirement, it also shows the situation of increased room temperature strength but insufficient plasticity and low-temperature impact resistance. It can be seen that maintaining Cr at the corresponding content level and controlling formula a within a certain range can obtain a good match between room temperature mechanical properties and low-temperature impact resistance. Appropriate Cr content and formula a have an important impact on obtaining the corresponding room temperature mechanical properties, low-temperature toughness, and ductile-brittle transition temperature.

[0027] For steel grades 14 - 19, the content of Nb was adjusted. It can be seen from the test results in Table 2 that when the Nb content is too low (steel grades 16, 19) or Nb is not added (steel grade 18), the strength decreases to varying degrees. The strength of steel grade 18 even fails to meet the requirements of the present invention. At the same time, due to the low addition amount of Nb or no addition of Nb, the low-temperature impact resistance of the cast steel has deteriorated significantly, and both the low-temperature toughness and the ductile-brittle transition temperature cannot meet the requirements of the present invention. For steel grade 14, the Nb content is too high. Although the strength is improved, the plasticity and low-temperature impact resistance of the cast steel decrease significantly. Although the Nb content of steel grades 15 and 17 meets the invention requirements, formula b is not within the invention requirements. When formula b is lower than the invention lower limit (steel grade 15), both the room-temperature mechanical properties and the low-temperature impact resistance of the cast steel deteriorate. When formula b is higher than the invention upper limit (steel grade 17), the room-temperature plasticity and low-temperature impact resistance of the cast steel also decrease significantly and cannot meet the invention requirements. Therefore, by controlling Nb and formula b within appropriate ranges, the present invention plays a prominent role in obtaining an excellent match between low-temperature mechanical properties and low-temperature impact resistance.

[0028] For steel grades 20 - 25, the content of Zr was adjusted. For cast steels with low Zr addition amounts (steel grades 21, 25) or no Zr addition (steel grade 20), their room-temperature strength is significantly insufficient. For cast steels with too high Zr addition amounts (steel grade 22), although the room-temperature strength is improved, the low-temperature toughness and the ductile-brittle transition temperature are not ideal and cannot meet the invention requirements. Although the Zr content of steel grades 23 and 24 meets the invention requirements, formula a and formula b do not meet the invention requirements. The test results show that when formula a and formula b are lower than the invention lower limit requirements (steel grade 23), although the low-temperature impact resistance of the cast steel is excellent, the room-temperature strength is insufficient. When formula a and formula b are higher than the invention upper limit requirements (steel grade 24), although the room-temperature strength of the cast steel is excellent, the low-temperature impact resistance decreases significantly. It is not difficult to conclude that by controlling Zr and formula a, formula b within appropriate ranges, it is possible to ensure that the cast steel has both excellent room-temperature mechanical properties and low-temperature impact resistance.

[0029] The content of each element of steel grades 1 - 7, as well as formula a and formula b, all meet the requirements of the present invention. It is confirmed from the test results in Table 2 that these cast steels have an excellent match between room-temperature mechanical properties and low-temperature impact resistance. It is confirmed that the cast steel prepared by the present invention through composition optimization and heat treatment processes of normalizing + quenching + tempering can meet the working conditions requirements of railway freight car parts in high-latitude and alpine regions with heavy loads.

[0030] The description of the above embodiments is only used to help understand the method and its core idea of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made to the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

[0031] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but rather should be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A low-temperature impact-resistant cast steel, characterized in that, The chemical composition of the low-temperature impact-resistant cast steel is as follows by weight percentage: C: 0.15 - 0.20%, Si: 0.22 - 0.39%, Mn: 0.88 - 1.35%, Ni: 0.35 - 0.55%, Cr: 0.10 - 0.19%, Nb: 0.02 - 0.06%, Zr: 0.008 - 0.021%, P ≤ 0.02%, S ≤ 0.02%, and the rest is Fe and inevitable impurities; moreover, the mass percentage contents [Cr], [Nb], [Zr] of Cr, Nb, Zr satisfy the following content relationship: 0.21 ≤ [Cr] + 9.57[Zr] ≤ 0.33; 0.071 ≤ [Nb] + 4.22[Zr] ≤ 0.

12.

2. The low-temperature impact-resistant cast steel according to claim 1, characterized in that The room-temperature mechanical properties of the low-temperature impact-resistant cast steel are: tensile strength above 620 MPa, yield strength above 415 MPa, and elongation above 27.5%.

3. The low-temperature impact-resistant cast steel according to any one of claims 1-2, characterized in that, The impact toughness KV2 of the low-temperature impact-resistant cast steel at -60 °C is above 65 J.

4. The low-temperature impact-resistant cast steel according to any one of claims 1-3, characterized in that, The ductile-brittle transition temperature of the low-temperature impact-resistant cast steel is lower than -88 °C.

5. The low-temperature impact-resistant cast steel according to any one of claims 1-4, characterized in that, The low-temperature impact-resistant cast steel is in a heat-treated state.

6. The preparation method of the low-temperature impact-resistant cast steel according to any one of claims 1-5, characterized in that It includes the following steps: smelting to obtain molten steel with corresponding components, casting the molten steel into a mold to obtain cast steel, and heat-treating the cast steel.

7. The preparation method of the low-temperature impact-resistant cast steel according to claim 6, wherein, The heat treatment sequentially includes normalizing, quenching, and tempering.

8. The preparation method of the low-temperature impact-resistant cast steel according to any one of claims 6-7, characterized in that The normalizing temperature is 950 - 980 °C, the quenching temperature is 880 - 910 °C, and the tempering temperature is 610 - 640 °C.

9. Application of the low-temperature impact-resistant cast steel according to claims 1 - 5 in railway freight car parts.

10. Application of the low-temperature impact-resistant cast steel prepared by the preparation method of the low-temperature impact-resistant cast steel according to claims 6 - 8 in railway freight car parts.

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