Low-yield-ratio steel for railway locomotives and preparation method of low-yield-ratio steel

Abstract

The invention discloses low-yield-ratio steel for a railway locomotive and a preparation method of the low-yield-ratio steel. The low-yield-ratio steel comprises the following components in percentage by mass: 0.071%-0.099% of C, 0.11%-0.21% of Si, 1.01%-1.70% of Mn, less than or equal to 0.025% of P, less than or equal to 0.008% of S, 0.11%-0.40% of Cr, 0.008%-0.04% of Zr, 0.012%-0.060% of Nb, 0.010%-0.040% of Als, 0.040%-0.09% of Nb + Zr and the balance of Fe and inevitable impurities. The steel for the railway locomotive, which is produced by adopting a C-Mn-Nb-Zr component system design, compositely adding Cr and Si elements and combining a corresponding production process, has the advantages of low yield ratio, high strength, excellent toughness and good plate shape, and good matching of comprehensive performance is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of materials and metallurgy, and more particularly to a low-yield ratio steel for railway locomotives and a preparation method thereof. Background Art

[0002] Domestic railway locomotives are mainly divided into diesel locomotives and electric locomotives (which can be divided into passenger and freight according to their use), among which electric locomotives are the main ones.

[0003] The Harmony series is the most classic electric locomotive. It utilizes an internationally advanced modular design approach based on functional systems, enhancing product flexibility and ensuring high levels of standardization, serialization, maintainability, and usability. The locomotive body utilizes a welded, integrally loaded frame structure, comprising a chassis, side walls, roof, driver's cab at both ends, and a driver's partition. Railway locomotives operate throughout the country, subjecting them to complex and ever-changing environments. This places high demands on their operational stability and safety. Locomotive manufacturing materials are also evolving towards easier welding, superior toughness, good plasticity, and a low yield-to-strength ratio. The performance and quality of existing steel plates are no longer adequate for these new technical requirements.

[0004] Therefore, it is necessary to develop a low yield ratio steel plate for railway locomotives to meet the development requirements of railway locomotive steel in my country.

[0005] Chinese patent CN105525210A discloses a low yield ratio Q390GJ construction steel plate and its production method, the chemical composition of which is: C: 0.15-0.17%, Si: 0.30-0.40%, Mn: 1.45-1.55%, P≤0.015%, S≤0.010%, Nb: 0.03-0.04%, V: 0.03-0.04%, Ti: 0.01-0.02%, Als: 0.020-0.040%, and the rest is Fe and impurities. Chinese patent CN115323271A discloses a 390 MPa yield strength, low yield ratio, high low temperature toughness, and refractory hot-rolled steel plate and its preparation method. The chemical composition is: C: 0.04-0.07%, Si: 0.25-0.35%, Mn: 0.90-1.20%, P≤0.020%, S≤0.010%, Mo: 0.15-0.20%, Ti: 0.015-0.035%, Cr: 0.25-0.40%, V: 0.020-0.025%, Nb: 0.04-0.07%, and the balance is Fe and unavoidable impurities. Chinese patent CN116334478A discloses a low-yield-to-strength ratio bridge steel plate and its manufacturing method. Its chemical composition is as follows: C: 0.04-0.07%; Si: 0.25-0.40%; Mn: 1.60-1.80%; Alt: 0.02-0.04%; Nb: 0.045-0.055%; Ti: 0.01-0.02%; Cr: 0.10-0.30%; Ni: 0.10-0.30%; Cu: 0.10-0.30%; Mo: 0.10-0.30%; the balance being Fe and unavoidable impurities. All of these patents contain multiple precious metal elements such as Mo, Nb, and V, and the added amounts are high, resulting in high manufacturing costs. Chinese patent CN110846585A discloses a Q345C steel plate for railway bogies and a manufacturing method thereof, comprising the following components: C: 0.08-0.12%, Mn: 1.2-1.6%, Si: 0.20-0.40%, P: ≤0.030%, S: ≤0.010%, Nb: 0.010-0.030%, Ti: 0.010-0.020%, Al: 0.010-0.050%, with the remainder being Fe and impurities; the carbon equivalent is less than 0.44; however, the patent does not analyze the yield strength ratio. Summary of the Invention

[0006] The purpose of the present invention is to overcome the above-mentioned defects of the prior art and provide a low yield ratio steel for railway locomotives and a preparation method thereof.

[0007] To achieve the above object, the technical solution of the present invention is as follows: A low-yield ratio steel for railway locomotives comprises the following components in percentage by mass: C: 0.071%-0.099%, Si: 0.11%-0.21%, Mn: 1.01%-1.70%, P≤0.025%, S≤0.008%, Cr: 0.11%-0.40%, Zr: 0.008%-0.04%, Nb: 0.012%-0.060%, Als: 0.010%-0.040%, Nb+Zr: 0.040%-0.09%, and the balance being Fe and unavoidable impurities.

[0008] The present invention also discloses a method for preparing the low-yield ratio steel for railway locomotives, comprising the following steps: molten iron pretreatment, converter smelting, LF refining, continuous casting, heating, rolling, cooling and straightening; and is characterized in that: During the heating, the thickness of the continuous casting billet is 200 mm to 250 mm, the heating temperature is 1160° C. to 1190° C., the heating time is ≥1.5 min / mm, the total time in the furnace is ≤500 min, and the holding time in the soaking section is ≥50 min; In the rolling, a two-stage controlled rolling process is adopted, the starting temperature of rough rolling is 1085°C to 1180°C, the thickness of the intermediate billet is 40mm to 100mm, and the thickness of the intermediate billet is 6 to 7 times the thickness of the finished product, the starting temperature of finishing rolling is 990°C to 1090°C, the total deformation of the last two passes of finishing rolling is ≤15%, the deformation of the last pass is ≤5%, the waiting time before the last pass is 20s to 60s, and the final rolling temperature of finishing rolling is 790°C to 830°C; During the cooling process, air cooling is performed for 7s~20s after rolling, and water cooling is started when the temperature reaches 680℃~720℃. The shielding distance between the head and tail of the cooling system is ≥3m, the water ratio is 1.4~1.8, and the water volume is 150m 3 / h~180m 3 / h, open 4~9 groups of header water, the return temperature is 550℃~600℃; In the straightening, after laminar cooling, the straightening temperature is 450° C. to 500° C., the hot straightening is performed for 2 to 4 passes, and the reduction is 0.4 mm to 2.1 mm.

[0009] The implementation of the present invention will have the following beneficial effects: The present invention adopts a C-Mn-Nb-Zr component system design, compositely adds Cr and Si elements, and combines corresponding smelting, heating, rolling, cooling and straightening production processes to produce railway locomotive steel. The steel has a thickness of 6 mm to 16 mm and a width of 2000 mm to 3000 mm, a yield strength of 390 MPa or more, a tensile strength of 580 MPa or more, an elongation after fracture of 18% or more, a -40°C KV2 longitudinal impact energy of 37 J or more, a yield ratio of 0.75 or less, and has a low yield ratio, high strength, excellent toughness and good plate shape. The metallographic structure is ferrite + bainite + pearlite, achieving a good match of comprehensive performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 This is the metallographic structure diagram of the low yield ratio steel for railway locomotives according to Example 1 of the present invention. DETAILED DESCRIPTION

[0011] The present invention will be further described below with reference to specific examples, but the present invention is not limited thereto in any way.

[0012] The invention discloses a low-yield ratio steel for railway locomotives. The steel comprises the following components in percentage by mass: C: 0.071% to 0.099%, Si: 0.11% to 0.21%, Mn: 1.01% to 1.70%, P≤0.025%, S≤0.008%, Cr: 0.11% to 0.40%, Zr: 0.008% to 0.04%, Nb: 0.012% to 0.060%, Als: 0.010% to 0.040%, Nb+Zr: 0.040% to 0.09%, and the balance being Fe and unavoidable impurities.

[0013] Specifically, the present invention selects the above alloying element types and contents because: Carbon is the most effective solid solution strengthening element in steel, significantly improving the strength of steel plates. A higher carbon content significantly improves weldability, impact toughness, and plasticity. However, too low a carbon content not only results in lower strength but also increases smelting costs. Therefore, a reasonable carbon content is essential. In this invention, the carbon content is controlled within a range of 0.071% to 0.099%.

[0014] Si: It has a moderate solid solution strengthening effect, can improve strength, and is the main deoxidizing element in the steelmaking process. Too high a content of Si will reduce the plasticity and toughness of the steel, increase the surface oxide scale and make it difficult to remove. Its content is controlled at 0.11% to 0.21%.

[0015] Mn: An important solid-solution strengthening element that increases strength, enhances austenite stability, and improves hardenability. However, excessive manganese content can easily induce segregation in continuous casting slabs, hindering banded structure control and deteriorating weldability and toughness in the heat-affected zone. Manganese content should be controlled between 1.01% and 1.70%.

[0016] P: It is prone to segregation, increasing the cold brittleness of steel, detrimental to low-temperature toughness, and worsening the weldability and cold formability of steel plates. Considering steelmaking operability and steelmaking costs, the P content is controlled to no more than 0.025%.

[0017] S: It is a harmful element in steel, which makes the steel hot brittle, reduces the ductility and toughness of the steel, and deteriorates the weldability and atmospheric corrosion resistance. Its content is controlled at no more than 0.008%.

[0018] Cr: Cr can reduce the γ phase region of steel. Its solid solubility in γ-Fe is 20%, and it can be infinitely soluble in α-Fe. Cr increases the stability of supercooled austenite in austenite, delaying the transformation of austenite to ferrite, reducing the pearlite content, and increasing proeutectoid ferrite. Cr can also reduce the diffusion rate of carbon in steel, lowering the critical cooling rate of steel and improving the hardenability of steel. Its content is controlled at 0.11% to 0.40%.

[0019] Zr: A strong carbonitride-forming element, the small oxide inclusions formed by Zr in microalloyed steels can pin austenite grain boundaries and prevent excessive austenite grain growth. The ability of second-phase particles to pin austenite grains depends primarily on particle size and number. Larger particles have very limited control over austenite grain growth. Conversely, smaller particles can effectively pin austenite grain boundaries, preventing growth. A greater number of particles effectively suppresses austenite grain growth. When Zr is added to microalloyed steel and heated to a certain temperature, particles such as ZrN and ZrC precipitate. Smaller nanoparticles (less than 100 nm) strongly inhibit austenite grain growth. Furthermore, the uniform distribution of Zr carbonitrides also refines austenite grains, enabling ultrafine grains to be obtained even when deformed in the high-temperature austenite recrystallization zone. When Zr is added to low-alloy steel, its carbonitrides disperse and precipitate as the rolling temperature decreases. These can serve as ferrite nucleation sites, promoting the formation of ferrite, increasing the ferrite content, and preventing grain growth, thereby changing the structure and morphology of the microstructure. ZrN can also serve as nucleation sites for NbC and NbN, causing Nb-containing carbonitrides to exist as Nb-Zr composites, thus exerting a precipitation strengthening effect. Excessive Zr content increases smelting difficulty and costs, and its content is controlled within a range of 0.008% to 0.04%.

[0020] Nb is a strong carbide-forming element that can increase the recrystallization temperature and strongly inhibit grain growth during the recrystallization process. The resulting fine carbide particles refine the grains. During rolling in the non-recrystallization zone, Nb carbonitrides disperse and precipitate before the austenite-ferrite transformation, becoming ferrite nucleation sites. This allows ferrite to form at a relatively low undercooling, refines the grains, and exhibits precipitation strengthening. Nb can also precipitate in combination with Zr, increasing high-angle grain boundaries through grain refinement and improving low-temperature toughness. Nb is a precious metal element with a high cost, and its content is controlled within the range of 0.012% to 0.060%. Furthermore, controlling the Nb+Zr content within the range of 0.040% to 0.09% ensures a good interaction between the two elements, achieving a significant phase transformation and precipitation effect.

[0021] AlS: A deoxidizer in steel, it also effectively refines grains and improves steel toughness. Combining with nitrogen to form AlN, it can reduce or eliminate two aging phenomena that occur at lower temperatures. The aluminum content is controlled between 0.010% and 0.040%.

[0022] In a specific embodiment, the low yield ratio steel for railway locomotives has a thickness of 6 mm to 16 mm and a width of 2000 mm to 3000 mm.

[0023] In a specific embodiment, the low yield ratio steel for railway locomotives has a yield strength ≥390 MPa, a tensile strength ≥580 MPa, an elongation after fracture ≥18%, a -40°C KV2 longitudinal impact energy ≥37 J, and a yield ratio ≤0.75.

[0024] In a specific embodiment, the metallographic structure of the low yield ratio railway locomotive steel is a three-phase composite of ferrite, bainite and pearlite; wherein the area fraction of ferrite is 65% to 75%, the area fraction of bainite is 15% to 25%, and the area fraction of pearlite is 5% to 15%.

[0025] The present invention also discloses a method for preparing the low yield ratio railway locomotive steel as described in any embodiment of the present invention, comprising the following steps: molten iron pretreatment, converter smelting, LF refining, continuous casting, heating, rolling, cooling and straightening.

[0026] S1. In the molten iron pretreatment, the molten iron is desulfurized to S≤0.0015% to obtain desulfurized molten iron.

[0027] S2. During converter smelting, the desulfurized molten iron is smelted in a converter and steel is tapped. A top and bottom combined blowing process is adopted. To avoid high temperature increasing the oxygen content of the molten steel, the converter terminal temperature is 1600℃~1620℃ to reduce the oxidizing property of the molten steel.

[0028] In S3, LF refining, the molten steel discharged from the converter is subjected to LF refining treatment.

[0029] In a specific embodiment, in the early stage of LF refining, the argon blowing time is 10-15 minutes, the argon pressure is 0.7-1.0 MPa, and stirring is performed for 3-5 minutes. In the middle stage of LF refining, the argon blowing time is 10-15 minutes, aluminum and iron are added for enhanced deoxidation, the argon pressure is 0.9-1.4 MPa, stirring is performed for 2-3 minutes, and silicon-calcium wire is fed at a feeding rate of 490-520 m3. In the late stage of LF refining, the argon blowing time is 7-10 minutes, the argon pressure is 0.3-0.5 MPa, and the weak blowing time is 3.0-4.2 minutes to promote the floating of inclusions in the molten steel, thereby ensuring narrow composition control, moderate temperature, and low inclusion level control in the molten steel.

[0030] In S4, during continuous casting, the molten steel subjected to LF refining treatment is continuously cast to obtain a continuously cast ingot.

[0031] In a specific embodiment, dynamic soft reduction technology is adopted in continuous casting, and the soft reduction amount is 3.5mm~7.5mm to strictly control the central porosity and segregation to ensure the internal quality of the ingot; the tundish temperature is 1540℃~1550℃, which not only avoids the erosion of refractory materials due to excessive temperature and casting accidents, but also avoids the freezing of molten steel or poor ingot quality caused by excessively low temperature. The billet drawing speed is 1.21m / min~1.41m / min, which improves the production rhythm and saves production costs.

[0032] S5. During heating, the thickness of the continuous casting ingot is 200mm~250mm, the heating temperature is 1160℃~1190℃, the heating time is ≥1.5min / mm, the total time in the furnace is ≤500min, and the holding time in the soaking section is ≥50min. Controlling the heating temperature and the total time in the furnace can fully dissolve the alloy elements, ensure the uniformity of the composition, reduce the temperature difference between the surface and the core of the ingot, ensure the fluidity of the metal in the transverse and longitudinal directions of the ingot, and reduce the deformation resistance during the rolling process. Controlling the holding time in the soaking section can make the microstructure and composition of the ingot uniform, improve the plasticity of the steel plate after rolling, and avoid the increase of iron oxide scale on the surface of the continuous casting ingot due to too long a time and the heat loss defect. Subsequently, through rolling, cooling and straightening, low yield ratio steel for railway locomotives is obtained.

[0033] In a specific embodiment, during rolling, a two-stage controlled rolling process is adopted, the starting temperature of rough rolling is 1085℃~1180℃, the thickness of the intermediate billet is 40mm~100mm, and the thickness of the intermediate billet is 6 times~7 times the thickness of the finished product, the starting temperature of finishing rolling is 990℃~1090℃, the total deformation of the last two passes of finishing rolling is ≤15%, the deformation of the last pass is ≤5%, the waiting time before the last pass is 20s~60s, and the final rolling temperature of finishing rolling is 790℃~830℃.

[0034] Specifically, rough rolling is carried out at a high temperature to prepare for finishing rolling, and the thickness of the intermediate billet is more than 6 times that of the finished product, which can ensure that the compression ratio in the finishing rolling stage is sufficient, the grains are uniform and fine, and the low-temperature impact toughness of the steel plate is guaranteed; since the product of the present invention is a thin-gauge product, the start temperature of finishing rolling is controlled to be 990°C to 1090°C, which can ensure that the rolling process proceeds smoothly; the last two passes of finishing rolling are carried out under a small deformation amount, which can ensure the plate shape of the thin-gauge steel plate, and waiting for 20s to 60s before the last rolling pass can ensure the precipitation of a large amount of pro-eutectoid ferrite, reduce the yield strength ratio of the steel plate, and at the same time, the final rolling temperature can be controlled to be 790°C to 830°C. It is carried out at a lower temperature to avoid that the grains in the steel have sufficient energy to recover and grow, ensure that the grains are small and uniform, and improve strength and toughness.

[0035] In a specific embodiment, during cooling and straightening, air cooling is performed for 7s to 20s after rolling, and water cooling is started at 680℃ to 720℃, which can promote the further precipitation of proeutectoid ferrite and ensure a low yield strength ratio. The shielding distance between the head and tail of the cooling system is ≥3m, which can make the steel plate cool evenly and ensure the consistency of overall performance. The difference between the cooling rate of the lower surface and the upper surface of the thin-gauge steel plate is small. The water ratio (the ratio of the lower header to the upper header) is 1.4 to 1.8, which can reduce the overall lateral buckling of the thin-gauge narrow width medium and thick plates. In view of the fast cooling rate of thin-gauge medium and thick plates, the water volume is 150m 3 / h~180m 3 / h, opening 4~9 groups of header water can further improve the plate shape quality; the red-hot temperature is 550℃~600℃, which can ensure sufficient cooling, form bainite and ensure strength; after laminar cooling, the straightening temperature is 450℃~500℃, hot straightening is 2~4 times, and the reduction is 0.4mm~2.1mm, which ensures the straightness of the steel plate, eliminates the cold straightening process and improves production efficiency.

[0036] The following are specific embodiments The composition of the steel of the embodiment of the present invention is shown in Table 1, the smelting process parameters of the steel of the embodiment of the present invention are shown in Table 2, the heating process parameters of the steel of the embodiment of the present invention are shown in Table 3, the rolling process parameters of the steel of the embodiment of the present invention are shown in Table 4, the cooling and straightening process parameters of the steel of the embodiment of the present invention are shown in Table 5, the properties of the steel of the embodiment of the present invention are shown in Table 6, and the metallographic structure of the steel of the embodiment of the present invention is shown in Table 7.

[0037] The present invention is described in more detail below through examples.

[0038] Table 1 Chemical composition of examples (%)

[0039] Table 2 Example smelting process parameters

[0040] Table 3 Continuous casting slab heating process parameters in Example

[0041] Table 4 Rolling process parameters of the embodiment

[0042] Table 5 Cooling and straightening process parameters of the embodiment

[0043] Table 6 Performance indicators of the embodiment

[0044] Table 7 Metallographic structure of the examples

[0045] The above-described embodiments merely represent several implementation methods of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that a person skilled in the art would be able to make various modifications and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A low yield ratio steel for railway locomotives, characterized in that: The following components are included in mass percentage: C: 0.071%~0.099%, Si: 0.11%~0.21%, Mn: 1.01%~1.70%, P≤0.025%, S≤0.008%, Cr: 0.11%~0.40%, Zr: 0.008%~0.04%, Nb: 0.012%~0.060%, Als: 0.010%~0.040%, and Nb+Zr: 0.040%~0.09%, the balance is Fe and unavoidable impurities.

2. The low yield ratio steel for railway locomotive according to claim 1, characterized in that: The low yield ratio steel for railway locomotives has a thickness of 6 mm to 16 mm and a width of 2000 mm to 3000 mm.

3. The low yield ratio steel for railway locomotive according to claim 1, characterized in that: The low yield ratio steel for railway locomotives has a yield strength of ≥390 MPa, a tensile strength of ≥580 MPa, an elongation after fracture of ≥18%, a -40°C KV2 longitudinal impact energy of ≥37 J, and a yield ratio of ≤0.

75.

4. The low yield ratio steel for railway locomotive according to claim 1, characterized in that: The metallographic structure of the low-yield ratio railway locomotive steel is a three-phase composite of ferrite, bainite and pearlite; wherein the area fraction of the ferrite is 65% to 75%, the area fraction of the bainite is 15% to 25%, and the area fraction of the pearlite is 5% to 15%.

5. A method for preparing the low yield ratio railway locomotive steel according to any one of claims 1 to 4, characterized in that: The following steps are involved: Hot metal pretreatment, converter smelting, LF refining, continuous casting, heating, rolling, cooling and straightening; its characteristics are: During the heating, the thickness of the continuous casting billet is 200 mm to 250 mm, the heating temperature is 1160° C. to 1190° C., the heating time is ≥1.5 min / mm, the total time in the furnace is ≤500 min, and the holding time in the soaking section is ≥50 min; In the rolling, a two-stage controlled rolling process is adopted, the rough rolling start temperature is 1085°C to 1180°C, the intermediate billet thickness is 40mm to 100mm, and the intermediate billet thickness is 6 to 7 times the thickness of the finished product, the finishing rolling start temperature is 990°C to 1090°C, the total deformation of the last two passes of finishing rolling is ≤15%, the deformation of the last pass is ≤5%, the waiting time before the last pass is 20s to 60s, and the finishing rolling final temperature is 790°C to 830°C; During the cooling process, air cooling is performed for 7s~20s after rolling, and water cooling is started when the temperature reaches 680℃~720℃. The shielding distance between the head and tail of the cooling system is ≥3m, the water ratio is 1.4~1.8, and the water volume is 150m 3 / h~180m 3 / h, open 4~9 groups of header water, the return temperature is 550℃~600℃; In the straightening, after laminar cooling, the straightening temperature is 450° C. to 500° C., the hot straightening is performed for 2 to 4 passes, and the reduction is 0.4 mm to 2.1 mm.

6. The preparation method according to claim 5, characterized in that In the LF refining, in the early stage of LF refining, the argon blowing time is 10 min to 15 min, the argon pressure is 0.7 MPa to 1.0 MPa, and the stirring is 3 min to 5 min; in the middle stage of LF refining, the argon blowing time is 10 min to 15 min, aluminum and iron are added to strengthen deoxidation, the argon pressure is 0.9 MPa to 1.4 MPa, the stirring is 2 min to 3 min, and silicon calcium wire is fed, and the feeding amount of silicon calcium wire is 490 m3 to 520 m3; in the late stage of LF refining, the argon blowing time is 7 min to 10 min, the argon pressure is 0.3 MPa to 0.5 MPa, and the weak blowing time is 3.0 min to 4.2 min.

7. The preparation method according to claim 5, characterized in that During the continuous casting, a dynamic soft reduction technology is adopted, with a soft reduction amount of 3.5 mm to 7.5 mm; the tundish temperature is 1540° C. to 1550° C., and the casting speed is 1.21 m / min to 1.41 m / min.

8. The preparation method according to claim 5, characterized in that In the molten iron pretreatment, the molten iron is desulfurized to S≤0.0015% to obtain desulfurized molten iron; During the converter smelting, the desulfurized molten iron is subjected to converter smelting and tapping, and a top and bottom composite blowing process is adopted, with the converter terminal temperature being 1600° C. to 1620° C.

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