Economical weathering steel for railway freight cars with a yield strength of 450 MPa and its production method

Abstract

A weathering steel with a yield strength of 450 MPa for economical railway freight cars comprises the following components by weight: C: 0.02-0.12%, Si: 0.10-0.60%, Mn: 0.3-1.0%, P: 0.02-0.15%, S: ≤0.005%, Cu: 0.10-0.55%, and Ti: 0.05-0.15%, and simultaneously satisfies the following two equations: Cu+2P+1.5Ti≥0.65%, and Cu / (2Ti+P)≤5. The steel is produced by conventional smelting and continuous casting into billets; heating the billets; rough rolling; finish rolling; rapid cooling; and coiling. The present invention ensures that the yield strength is ≥450MPa, the tensile strength is ≥550MPa, the elongation is ≥22%, and the Kv2 at -20°C is ≥100J. The corrosion rate is less than 90% of that of Q450NQR1, and there is no need to add precious alloys such as Cr and Ni, thereby significantly reducing the production cost of steel.

Description

Technical Field

[0001] The present invention relates to steel for railway vehicles and a production method thereof, and in particular to weathering steel with a yield strength of 450 MPa for economical railway freight cars and a production method thereof. Background Art

[0002] my country's railway vehicle industry currently widely uses weathering steel grades such as Q450NQR1, as specified in the TB / T 1979 standard, for domestic and export freight cars. On the one hand, the addition of precious alloying elements such as Cr and Ni to these materials results in high production costs, significantly limiting the profitability and market competitiveness of railway vehicle manufacturers. On the other hand, railway freight cars operate outdoors for long periods of time, exposed to the harsh environment of air, transport media such as coal, antifreeze, water, and mechanical impact. This leads to rapid corrosion of the Q450NQR1 material, resulting in a short vehicle lifespan. Therefore, there is an urgent need to develop a low-cost, highly corrosion-resistant weathering steel for railway freight cars.

[0003] CN202110317342.6 and CN202010153281.X respectively disclose an economical low-yield ratio hot-rolled weathering steel plate and its manufacturing method, and a titanium micro-alloyed economical high-strength weathering steel and its production method, but both require the addition of a certain amount of precious alloy elements such as Cr and Ni; CN200510045624.6 discloses an economical weathering steel, but its yield strength is only 345MPa, which cannot meet the heavy-load requirements of railway freight cars. Summary of the Invention

[0004] The present invention aims to overcome the shortcomings of the prior art, which have many alloy elements and expensive elements added, resulting in high production costs. The present invention provides a 450MPa yield strength economical weathering steel for railway freight cars, which can significantly reduce production costs while ensuring a yield strength of 450MPa or higher, a tensile strength of 550MPa or higher, and an elongation of 22% or higher, and a Kv2 of 100J at -20°C and a corrosion rate of less than 90% of that of Q450NQR1.

[0005] Measures to achieve the above objectives:

[0006] Disclosed is a 450 MPa yield strength economical weathering steel for railway freight cars. The steel comprises the following components and weight percentages: C: 0.02-0.12%, Si: 0.10-0.60%, Mn: 0.3-1.0%, P: 0.02-0.15%, S: ≤0.005%, Cu: 0.10-0.55%, and Ti: 0.05-0.15%. The steel also satisfies the following two equations: Cu+2P+1.5Ti≥0.65%, and Cu / (2Ti+P)≤5, with the remainder being Fe and impurities.

[0007] Preferably, the weight percentage of Cu is 0.16-0.48%.

[0008] Preferably, the weight percentage of P is 0.026-0.125%.

[0009] Preferably, the weight percentage content of Ti is 0.06-0.072%.

[0010] A method for producing an economical weathering steel for railway freight cars with a yield strength of 450 MPa, comprising the following steps:

[0011] 1) Conventional smelting and continuous casting into billets;

[0012] 2) Heating the ingot: Control the heating temperature of the ingot at 1180-1260°C;

[0013] 3) Perform rough rolling, with the starting rolling temperature not lower than 1100°C;

[0014] 4) Finish rolling is performed with the final rolling temperature being 820-890°C;

[0015] 5) Rapid cooling is performed at a cooling rate of 40 to 120°C / s to the coiling temperature;

[0016] 6) Coiling: Control the coiling temperature at 580-650°C.

[0017] Functions and mechanisms of the components and main processes in the present invention

[0018] Carbon is the most cost-effective alloying element for increasing steel strength. However, excessive carbon content significantly deteriorates steel's weldability and promotes pearlite transformation, thereby reducing the steel's corrosion resistance. The present invention employs an ultra-low carbon design to improve steel's weldability, inhibit pearlite formation, and enhance the steel's corrosion resistance and low-temperature impact toughness. The carbon content in the steel of the present invention is 0.02-0.12%.

[0019] Si accelerates carbon segregation toward austenite in steel, purifies ferrite, and prevents the formation of coarse carbides during cooling. Solid-solution silicon also allows for uniform extension and improves the steel's corrosion resistance. Furthermore, Si can inhibit the occurrence of "copper brittleness" defects on the surface of Ni-free copper-containing steel. However, excessive silicon content can form Fe2SiO4 during high-temperature rolling, increasing the adhesion of iron oxide scale and deteriorating the surface quality of the steel plate. Therefore, the Si content is controlled within a range of 0.10% to 0.60%.

[0020] Adding Mn to steel can not only improve the strength of the steel through solid solution strengthening of Mn, but also reduce the phase transition temperature of the steel, refine the grains, and improve the low-temperature toughness of the steel. The Mn content of the steel of the present invention is designed to be 0.3-1.0%.

[0021] S is a harmful element in steel. The generated sulfide inclusions seriously affect the low-temperature impact toughness of steel. Therefore, the S content in steel should be reduced as much as possible to below 0.005%.

[0022] Cu is the most important alloying element for improving the corrosion resistance of steel. Cathodic contact between steel and secondary Cu precipitation on the surface promotes anodic passivation of the steel and forms a protective rust layer, significantly improving the corrosion resistance of the steel. However, excessive Cu content will reduce the toughness of the steel plate's weld heat-affected zone and cause "copper embrittlement" during the continuous casting and hot rolling processes. Therefore, the Cu content is controlled between 0.10 and 0.55%.

[0023] Phosphorus (P) promotes the formation of a dense rust layer on the steel surface, significantly improving the steel's weather resistance. It also inhibits the penetration of copper-rich phases into austenite grain boundaries, thereby suppressing copper brittleness. However, excessive P content can easily lead to segregation, deteriorating cold forming properties and low-temperature impact toughness. The P content in the steel of this invention is controlled to be between 0.02% and 0.15%.

[0024] Ti: Ti forms TiC second-phase particles in steel, hindering austenite grain growth, significantly refining austenite grains, increasing the austenite grain boundary area, and reducing the penetration depth of the liquid Cu-rich phase on the steel matrix surface into the austenite grain boundaries, thereby effectively suppressing the "copper brittleness" defect that is common in Ni-free Cu-containing steels. Furthermore, the addition of Ti exerts a certain strengthening effect. While maintaining the original strength level, the design value of C can be reduced. The C element is fixed as TiC, further suppressing the transformation of C-rich austenite to pearlite, forming a single-phase ferrite structure. This avoids galvanic corrosion between the different structures in conventional ferrite-pearlite steels, thereby further improving the steel's corrosion resistance. Furthermore, Ti can increase the density of the rust layer, further enhancing the steel's corrosion resistance. The Ti content in the present invention is designed to be 0.05-0.15%. Furthermore, to ensure corrosion resistance, the ratio Cu+2P+1.5Ti is controlled to be ≥ 0.65. To ensure surface quality of the steel plate and suppress copper brittleness, the ratio Cu / (2Ti+P) is controlled to be ≤ 2.5.

[0025] The present invention controls the heating temperature of the ingot to 1180-1260°C to ensure sufficient austenitization and solid solution of the Ti element. If the heating temperature is too low, austenitization and solid solution of Ti are inadequate; if the heating temperature is too high, the copper brittleness defect of Ni-free Cu-containing steel is aggravated.

[0026] The reason why the starting temperature of the rough rolling is controlled to be ≥1100° C. in the present invention is that a uniform and fine structure can be obtained within this temperature range. When the starting temperature of the rough rolling is too low, mixed crystals will be generated.

[0027] The reason for controlling the final rolling temperature between 820°C and 890°C in this invention is to obtain a uniform and fine transformation structure, thereby improving strength and toughness. If the final rolling temperature is too low, rolling enters the two-phase region, resulting in mixed crystals; if the final rolling temperature is too high, the grains will be coarse and the toughness will be reduced.

[0028] The present invention controls the cooling rate to 40-120°C / s to achieve a good match between strength and elongation. If the cooling rate is too low, the microstructure will not be fine enough and the steel will be insufficiently strong; if the cooling rate is too high, the strength will be too high and the elongation will be too low.

[0029] The coiling temperature in the present invention is controlled between 580°C and 650°C to obtain sufficient fine TiC precipitates. If the coiling temperature is too low, the dissolved Ti element cannot be fully precipitated; if the coiling temperature is too high, the precipitates become coarse, which will reduce the impact toughness of the steel.

[0030] Compared with the prior art, the present invention ensures that the yield strength is ≥450MPa, the tensile strength is ≥550MPa, the elongation is ≥22%, and the Kv2 at -20°C is ≥100J, the corrosion rate is less than 90% of that of Q450NQR1, and there is no need to add precious alloys such as Cr and Ni, thereby significantly reducing the production cost of steel. DETAILED DESCRIPTION

[0031] The present invention is described in detail below:

[0032] Table 1 is a list of chemical compositions of various embodiments and comparative examples of the present invention;

[0033] Table 2 is a list of main process parameters of various embodiments and comparative examples of the present invention;

[0034] Table 3 is a table of performance test results of various embodiments and comparative examples of the present invention.

[0035] Each embodiment of the present invention is produced according to the following steps

[0036] 1) Conventional smelting and continuous casting into billets;

[0037] 2) Heating the ingot: Control the heating temperature of the ingot at 1180-1260°C;

[0038] 3) Perform rough rolling, with the starting rolling temperature not lower than 1100°C;

[0039] 4) Finish rolling is performed with the final rolling temperature being 820-890°C;

[0040] 5) Rapid cooling is performed at a cooling rate of 40 to 120°C / s to the coiling temperature;

[0041] 6) Coiling: Control the coiling temperature at 580-650°C.

[0042] Table 1 Chemical composition list of various embodiments and comparative examples of the present invention (wt%)

[0043]

[0044]

[0045] Table 2 List of main process parameters of various embodiments of the present invention and comparative examples

[0046]

[0047] Table 3 Mechanical properties test results of various embodiments of the present invention and comparative examples

[0048]

[0049]

[0050] Description: Corrosion test conditions: Refer to "TB / T 2375-93 Test method for periodic immersion corrosion of weathering steel for railway use" for cyclic immersion corrosion test. The solution used is 0.01 mol / L NaHSO3, the temperature is 45±2℃, the relative humidity is 70±5%, and the test time is 72h.

[0051] The test results are shown in Table 3. The corrosion test results show that the corrosion rate of the steel of the present invention is less than 90% of that of Q450NQR1, a common weathering steel used for railway freight cars, indicating significantly higher corrosion resistance than Q450NQR1 and excellent corrosion resistance. Furthermore, the steel exhibited yield strengths exceeding 450 MPa, tensile strengths exceeding 550 MPa, and elongations exceeding 22%. The steel also passed the 180° bend test at B = 20 mm and d = 2a, and Kv2 at -20°C exceeding 100 J.

[0052] This specific implementation is only the best example and is not a restrictive implementation of the technical solution of the present invention.

Claims

1. A 450 MPa yield strength economical weathering steel for railway freight cars, comprising the following components and weight percentages: C: 0.02-0.12%, Si: 0.10-0.60%, Mn: 0.3-1.0%, P: 0.02-0.15%, S: ≤0.005%, Cu: 0.10-0.55%, Ti: 0.05-0.15%, and satisfying the following two equations: 0.65% ≤ Cu + 2P + 1.5Ti ≤ 0.68%, Cu / (2Ti + P) ≤ 2.29, with the remainder being Fe and impurities; Production method: 1) Conventional smelting and continuous casting into billets; 2) Heating the ingot: Control the heating temperature of the ingot at 1180-1260℃; 3) Perform rough rolling, with the starting rolling temperature not lower than 1100°C; 4) Finish rolling is carried out with the final rolling temperature at 820-890°C; 5) Rapid cooling is performed at a cooling rate of 40 to 120 ° C / s to the coiling temperature; 6) Coiling: Control the coiling temperature at 580-650℃.

2. The economical weathering steel for railway freight cars with a yield strength of 450 MPa according to claim 1, characterized in that: The weight percentage of Cu is 0.16~0.48%.

3. The economical weathering steel for railway freight cars with a yield strength of 450 MPa according to claim 1, characterized in that: The weight percentage of P is 0.026~0.125%.

4. The economical weathering steel for railway freight cars with a yield strength of 450 MPa according to claim 1, characterized in that: The weight percentage content of Ti is 0.06~0.072%.

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