Low chromium ferritic stainless steel for highway guardrails and preparation method thereof

Abstract

The invention discloses a low-chromium ferritic stainless steel for highway guardrails and a preparation method thereof. The chemical composition of the stainless steel is: C≤0.030%, Si≤0.45%, Mn≤1.20%, P≤0.015%, S≤0.005%, Cr 11.50-13.50%, Ni≤0.60%, N≤0.015%, B 0.001-0.003%, and the balance is Fe. The preparation method comprises: preparing a continuous casting slab, heating it, hot rolling it, and coiling it to produce a hot-rolled coil, wherein the heating temperature is controlled at 1130-1170°C, the furnace dwell time is controlled at 200-240 minutes, the final rolling temperature is controlled at 880-920°C, and the coiling temperature is controlled at 660-700°C; the hot-rolled coil is subjected to a high-strength continuous annealing heat treatment, wherein the preheating temperature is controlled at 500-650°C, the annealing temperature is controlled at 850-880°C, the wire speed is controlled at 20-40 m / min, and the cooling rate is controlled at 50-70°C / s. By optimizing the alloy composition, controlling the rolling process parameters, and performing a high-strength continuous annealing heat treatment, the present invention achieves a ferrite + martensite composite structure in the material matrix, combining high strength, good ductility and low-temperature impact toughness, meeting the technical requirements for highway guardrail steel.

Description

Technical Field

[0001] The invention belongs to the technical field of ferritic stainless steel, and in particular relates to low-chromium ferritic stainless steel for highway guardrails and a preparation method thereof. Background Art

[0002] Expressways are a key symbol of national modernization. China's road network is already among the largest in the world, particularly in expressway mileage. China's expressways are undergoing a dramatic transformation from initial construction to a fully operational network. With new, large-scale road network planning and infrastructure construction about to begin nationwide, the scale of new highway construction, renovation, expansion, and optimization projects is enormous, creating a promising market for highway guardrails.

[0003] As my country's road traffic system continues to improve and the mileage of open highways continues to increase, the current number of cars is constantly rising, and logistics vehicles are showing a trend of becoming larger and more heavily loaded. Highway safety hazards are becoming increasingly prominent, and road traffic accidents are prone to occur frequently. As a basic safety facility on highways, corrugated beam guardrails are an important barrier to reduce the degree of personal injury after an accident, playing a positive and important role in promoting traffic safety. The corrugated beam guardrail on highways absorbs the kinetic energy of the vehicle involved in the accident through the plastic deformation of the guardrail structure itself during the collision, thereby achieving the purpose of stopping the vehicle. In order to prevent vehicles from running off the road and minimize the degree of injury to passengers, the guardrail material must have high strength, high toughness, and good dynamic impact resistance.

[0004] my country has clear technical requirements for steel used in highway guardrails: yield strength ≥ 235MPa, tensile strength ≥ 375MPa, elongation ≥ 26%, and impact energy (Akv) (longitudinal, -20°C to 20°C) ≥ 27J. However, Q235 carbon structural steel is currently widely used as the base material for corrugated beam guardrails. Due to its relatively low strength, Q235 steel does not fully meet the actual protection needs in collisions involving medium-sized, large, and heavy vehicles. Vehicles often overrun, ride over, or even climb over the guardrails. Furthermore, the hot-dip galvanizing process used for corrosion protection of carbon steel guardrails can cause serious environmental pollution and occupational health issues.

[0005] As my country's requirements for highway guardrail protection levels continue to rise, existing carbon steel guardrail materials face challenges with low strength and toughness, as well as environmental concerns stemming from the galvanizing process. To meet the urgent demands for upgrading highway guardrail safety and creating a green transportation system, there is an urgent need to develop high-strength, high-toughness, and environmentally friendly stainless steel for highway guardrails. Summary of the Invention

[0006] In order to solve the technical problems existing in the above-mentioned prior art, the present invention provides a low-chromium ferritic stainless steel for highway guardrails and a preparation method thereof.

[0007] The low-chromium ferritic stainless steel for highway guardrails provided by the present invention has a matrix structure of ferrite + martensite, and its chemical composition is controlled by mass percentage as follows: C≤0.030%, Si≤0.45%, Mn≤1.20%, P≤0.015%, S≤0.005%, Cr 11.50-13.50%, Ni≤0.60%, N≤0.015%, B 0.001-0.003%, and the balance is Fe.

[0008] Preferably, in the above-mentioned low-chromium ferritic stainless steel for highway guardrails, the chemical composition of the low-chromium ferritic stainless steel for highway guardrails is controlled as follows by mass percentage: C 0.020%, Si 0.35%, Mn 0.85%, P 0.005%, S 0.001%, Cr 12.00%, Ni 0.22%, N 0.010%, B 0.0018%, and the balance is Fe.

[0009] Preferably, in the above-mentioned low-chromium ferritic stainless steel for highway guardrails, the chemical composition of the low-chromium ferritic stainless steel for highway guardrails is controlled as follows by mass percentage: C 0.025%, Si 0.30%, Mn 0.90%, P 0.003%, S 0.001%, Cr 11.70%, Ni 0.20%, N 0.012%, B 0.0025%, and the balance is Fe.

[0010] Preferably, in the above-mentioned low-chromium ferritic stainless steel for highway guardrails, the chemical composition of the low-chromium ferritic stainless steel for highway guardrails is controlled as follows by mass percentage: C 0.022%, Si 0.40%, Mn 0.95%, P 0.004%, S 0.001%, Cr 11.85%, Ni 0.45%, N 0.010%, B 0.0020%, and the balance is Fe.

[0011] The method for preparing low-chromium ferritic stainless steel for highway guardrails provided by the present invention comprises the following steps:

[0012] S1: Continuously cast slabs are produced through a process of "molten iron pretreatment + K-OBM-S converter + VOD furnace + slab continuous casting + grinding", wherein the chemical composition of the continuously cast slabs is controlled by mass percentage as follows: C ≤ 0.030%, Si ≤ 0.45%, Mn ≤ 1.20%, P ≤ 0.015%, S ≤ 0.005%, Cr 11.50-13.50%, Ni ≤ 0.60%, N ≤ 0.015%, B 0.001-0.003%, and the balance is Fe;

[0013] S2: The continuous casting slab is heated, descaled, rough rolled, finish rolled, and coiled to produce a hot rolled coil with a thickness of 2.0 to 6.0 mm. The heating temperature of the continuous casting slab is controlled at 1130 to 1170°C, the furnace dwell time is controlled at 200 to 240 minutes, the final rolling temperature is controlled at 880 to 920°C, and after rolling, it is subjected to laminar cooling. The coiling temperature is controlled at 660 to 700°C.

[0014] S3: The hot-rolled coil is subjected to a high-strength continuous annealing heat treatment in a continuous annealing furnace. The high-strength continuous annealing heat treatment includes: the hot-rolled coil is fed into the preheating zone of the annealing furnace for preheating, and the preheating temperature is controlled at 500-650°C; after preheating, the steel plate is fed into the process temperature control zone of the annealing furnace for annealing, and the annealing temperature is controlled at 850-880°C, and the wire speed is controlled at 20-40m / min; after annealing, the steel plate passes through the cooling zone of the annealing furnace and is cooled to room temperature at an average cooling rate of 50-70°C / s.

[0015] Preferably, in the above-mentioned method for preparing low-chromium ferritic stainless steel for highway guardrail:

[0016] In step S1, the chemical composition of the continuous casting slab is controlled as follows by mass percentage: C 0.020%, Si 0.35%, Mn 0.85%, P 0.005%, S 0.001%, Cr 12.00%, Ni 0.22%, N 0.010%, B 0.0018%, and the balance is Fe;

[0017] In step S2, the heating temperature of the continuous casting slab is controlled at 1145°C and the finishing rolling temperature is controlled at 910°C;

[0018] In step S3, the preheating temperature is controlled at 580°C, the annealing temperature is controlled at 860°C, the wire speed is controlled at 35 m / min, and the cooling rate is controlled at 62°C / s.

[0019] Preferably, in the above-mentioned method for preparing low-chromium ferritic stainless steel for highway guardrail:

[0020] In step S1, the chemical composition of the continuous casting slab is controlled as follows by mass percentage: C 0.025%, Si 0.30%, Mn 0.90%, P 0.003%, S 0.001%, Cr 11.70%, Ni 0.20%, N 0.012%, B 0.0025%, and the balance is Fe;

[0021] In step S2, the heating temperature of the continuous casting slab is controlled at 1150°C and the finishing rolling temperature is controlled at 920°C;

[0022] In step S3, the preheating temperature is controlled at 615°C, the annealing temperature is controlled at 860°C, the line speed is controlled at 25 m / min, and the cooling rate is controlled at 56°C / s.

[0023] Preferably, in the above-mentioned method for preparing low-chromium ferritic stainless steel for highway guardrail:

[0024] In step S1, the chemical composition of the continuous casting slab is controlled as follows by mass percentage: C 0.022%, Si 0.40%, Mn 0.95%, P 0.004%, S 0.001%, Cr 11.85%, Ni 0.45%, N 0.010%, B 0.0020%, and the balance is Fe;

[0025] In step S2, the heating temperature of the continuous casting slab is controlled at 1160°C and the finishing rolling temperature is controlled at 880°C;

[0026] In step S3, the preheating temperature is controlled at 600°C, the annealing temperature is controlled at 875°C, the wire speed is controlled at 27 m / min, and the cooling rate is controlled at 60°C / s.

[0027] The low-chromium ferritic stainless steel for highway guardrails and the preparation method thereof of the present invention have the following advantages and beneficial effects:

[0028] Through the optimization design of alloy composition, a new low-chromium ferritic stainless steel grade TGR-H for highway guardrails was obtained; by reasonably controlling the rolling process parameters, it is ensured that the continuous casting ingot generates a single austenite structure in the high-temperature stage; through the high-intensification continuous annealing heat treatment process, the proportion of martensite phase in the steel is regulated, so that the annealed steel plate matrix becomes a complex phase structure of ferrite + a certain proportion of martensite. The material has high strength, good plasticity and toughness, and low-temperature impact toughness, with excellent comprehensive performance. It can be used in the production of highway guardrails without the implementation of galvanizing anti-corrosion process, and fully meets the technical requirements for highway guardrail steel: yield strength ≥235MPa, tensile strength ≥375MPa, elongation ≥26%, impact energy Akv (longitudinal, -20℃~20℃) ≥27J. Therefore, the highway guardrail produced using the low-chromium ferritic stainless steel for highway guardrail of the present invention can achieve a lightweight design of the guardrail structure while ensuring high strength and high toughness, and has the beneficial effects of saving materials, energy saving and consumption reduction, cost reduction and green environmental protection. Under the huge market scale effect of highway guardrails in the future, it will produce significant economic benefits, social benefits and environmental benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0030] Figure 1 This is a metallographic microstructure diagram of the low-chromium ferrite stainless steel for highway guardrails after high-strength continuous annealing of the present invention;

[0031] Figure 2 This is the EBSD phase ratio analysis result of the low-chromium ferritic stainless steel for highway guardrails after high-strength continuous annealing of the present invention. DETAILED DESCRIPTION

[0032] To make the objectives, technical solutions, and advantages of the present invention more clear, the following will provide a clear and complete description of the technical solutions of the present invention in conjunction with specific embodiments of the present invention and the accompanying drawings. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0033] The low-chromium ferritic stainless steel for highway guardrails of the present invention is a new type of low-chromium ferritic stainless steel (Taiyuan Iron and Steel Co., Ltd. grade TGR-H) developed by optimizing alloy composition, and its matrix structure is ferrite + a certain proportion of martensite. Its chemical composition is controlled by mass percentage as follows: C≤0.030%, Si≤0.45%, Mn≤1.20%, P≤0.015%, S≤0.005%, Cr11.50-13.50%, Ni≤0.60%, N≤0.015%, B 0.001-0.003%, and the balance is Fe.

[0034] The chemical composition of the low chromium ferritic stainless steel for highway guardrails of the present invention is controlled according to the above mass percentages to achieve optimized alloy composition because:

[0035] (1) Carbon (C): As an austenite-forming element, C can improve the hardenability of steel. As the carbon content in steel increases, the strength and hardness increase, while the plasticity and toughness of the steel decrease, and the corrosion resistance and weldability deteriorate. Therefore, in the low-chromium ferritic stainless steel of the present invention, the mass percentage of C is controlled to C≤0.030%, thereby ensuring that the material has both high strength and toughness and good corrosion resistance and weldability by adopting a low-carbon design.

[0036] (2) Nickel (Ni): As an austenite-forming element, Ni can increase the proportion of high-temperature austenite phase, significantly improve the plasticity and toughness of stainless steel, and enhance the formability and weldability of the material. However, Ni is a precious metal and has a high cost. Therefore, in the low-chromium ferritic stainless steel of the present invention, Ni is added only as a trace alloying element, and its mass percentage content is controlled to be Ni≤0.60%, thereby taking into account the material cost.

[0037] (3) Manganese (Mn): As a weak austenite-forming element, Mn can be appropriately increased to replace expensive and scarce Ni. However, as the main deoxidizing element in ferritic stainless steel, Mn generally needs to be controlled at a low level. Moreover, Mn and S have a strong affinity, which easily forms MnS inclusions and deteriorates material properties. Therefore, in the low chromium ferritic stainless steel of the present invention, the mass percentage of Mn is controlled to Mn≤1.20%.

[0038] (4) Nitrogen (N): As an austenite-forming element, N can play a certain role in solid solution strengthening. However, the solubility of N in ferrite is very low, and it is easy to form nitrides, thereby reducing the corrosion resistance. A high N content will also reduce the ductile-brittle transition temperature, thereby affecting the low-temperature impact toughness of the material. Therefore, in the low-chromium ferritic stainless steel of the present invention, the mass percentage of N is controlled to N≤0.015%.

[0039] (5) Silicon (Si): Si is a strong ferrite-forming element and is added as a deoxidizer. However, excessive addition will reduce the plasticity of the material. Therefore, in the low-chromium ferritic stainless steel of the present invention, the mass percentage content of Si is controlled to Si≤0.45%.

[0040] (6) Boron (B): Adding a trace amount of Boron can improve the hot working properties of the material. A too low Boron content has little effect on the hot working properties, while a too high Boron content will cause low-melting-point borides to precipitate at the martensite-ferrite phase boundary, reducing the phase boundary bonding force and being detrimental to the hot working properties. Therefore, in the low chromium ferritic stainless steel of the present invention, the mass percentage content of B is controlled to be 0.001 to 0.003%.

[0041] (7) Phosphorus (P) and sulfur (S): P and S, as impurity elements in steel, will significantly deteriorate the hot working properties of the material. Their content should be reduced as much as possible. In addition, S will also reduce the pitting corrosion resistance of ferritic stainless steel. Its content needs to be strictly controlled. Therefore, in the low chromium ferritic stainless steel of the present invention, the mass percentage content of P is controlled to P≤0.015%, and the mass percentage content of S is controlled to S≤0.005%.

[0042] The method for preparing low-chromium ferritic stainless steel for highway guardrails of the present invention comprises the following steps:

[0043] S1: Continuously cast slabs are produced through a process of "molten iron pretreatment + K-OBM-S converter + VOD furnace + slab continuous casting + grinding", wherein the chemical composition of the continuously cast slabs is controlled by mass percentage as follows: C ≤ 0.030%, Si ≤ 0.45%, Mn ≤ 1.20%, P ≤ 0.015%, S ≤ 0.005%, Cr 11.50-13.50%, Ni ≤ 0.60%, N ≤ 0.015%, B 0.001-0.003%, and the balance is Fe;

[0044] S2: The continuous casting slab is heated, descaled, rough rolled, finish rolled, and coiled to produce a hot rolled coil with a thickness of 2.0-6.0 mm. The heating temperature of the continuous casting slab is controlled at 1130-1170°C, and the furnace dwell time is controlled at 200-240 minutes to ensure that the continuous casting slab forms a single austenite structure at the high temperature stage. The final rolling temperature is controlled at 880-920°C, and the slab is subjected to laminar cooling after rolling. The coiling temperature is controlled at 660-700°C.

[0045] S3: The hot rolled coil is subjected to a high-strength continuous annealing heat treatment in a continuous annealing furnace. The high-strength continuous annealing heat treatment includes:

[0046] S31: The hot rolled coil is fed into the preheating zone of the annealing furnace for preheating, and the preheating temperature is controlled at 500-650°C;

[0047] S32: After preheating, the steel plate is sent to the process temperature control zone of the annealing furnace for annealing. The annealing temperature is controlled at 850-880°C and the wire speed is controlled at 20-40m / min.

[0048] S33: After annealing, the steel plate passes through the cooling zone of the annealing furnace and is cooled to room temperature at an average cooling rate of 50 to 70°C / s.

[0049] Low-chromium ferritic stainless steel for highway guardrails is thus produced, and the standard range of its chemical composition is as follows by mass percentage: C≤0.030%, Si≤0.45%, Mn≤1.20%, P≤0.015%, S≤0.005%, Cr 11.50-13.50%, Ni≤0.60%, N≤0.015%, B 0.001-0.003%, and the balance is Fe.

[0050] In the method for preparing low-chromium ferritic stainless steel for highway guardrails of the present invention, a new low-chromium ferritic stainless steel grade TGR-H for highway guardrails is obtained by optimizing the alloy composition in step S1; by controlling the hot rolling process parameters in step S2, it is ensured that the continuous casting ingot generates a single austenite structure in the high-temperature stage; through the high-strength continuous annealing heat treatment process in step S3, more specifically, by controlling the annealing temperature to 850-880°C, the stainless steel is made to have a ferrite + austenite two-phase region at a high temperature of 850-880°C, and then during the cooling process of the steel plate, the high-temperature austenite is transformed into a martensite phase, thereby making the annealed steel plate matrix a complex phase structure of ferrite + a certain proportion of martensite, such as Figure 1 and Figure 2 shown.

[0051] The low-chromium ferritic stainless steel for highway guardrails and its preparation method of the present invention are described below with reference to specific embodiments.

[0052] Example 1

[0053] The method for preparing low-chromium ferritic stainless steel for highway guardrails according to Example 1 of the present invention comprises the following steps:

[0054] S1: Continuously cast slabs are produced by the process of "hot metal pretreatment + K-OBM-S converter + VOD furnace + slab continuous casting + grinding", wherein the chemical composition of the continuously cast slabs is controlled as follows by mass percentage: C 0.020%, Si 0.35%, Mn 0.85%, P 0.005%, S 0.001%, Cr 12.00%, Ni 0.22%, N 0.010%, B 0.0018%, and the balance is Fe;

[0055] S2: The continuous casting slab is heated, descaled, rough rolled, finish rolled, and coiled to produce a hot rolled coil with a thickness of 3.7 mm and a width of 1545 mm. The heating temperature of the continuous casting slab is controlled at 1145°C and the finishing rolling temperature is controlled at 910°C.

[0056] S3: The hot rolled coil is subjected to a high-strength continuous annealing heat treatment in a continuous annealing furnace. During the high-strength continuous annealing heat treatment, the preheating temperature is controlled at 580°C, the annealing temperature is controlled at 860°C, the wire speed is controlled at 35m / min, and the cooling rate is controlled at 62°C / s.

[0057] The resulting low-chromium ferritic stainless steel for highway guardrails has the following chemical composition by mass: C 0.020%, Si 0.35%, Mn 0.85%, P 0.005%, S 0.001%, Cr 12.00%, Ni 0.22%, N 0.010%, B 0.0018%, with the balance being Fe. Metallographic examination and EBSD phase ratio analysis revealed a matrix structure of ferrite with a certain proportion of martensite. The mechanical properties of the material were tested as follows: yield strength = 360 MPa, tensile strength = 558 MPa, elongation = 28%, room temperature impact energy (sample size 3.7×10×55 mm, longitudinal) Akv = 66 J, -20°C impact energy (sample size 3.7×10×55 mm, longitudinal) Akv = 52 J, which fully meet the technical requirements for highway guardrail steel: yield strength ≥ 235 MPa, tensile strength ≥ 375 MPa, elongation ≥ 26%, impact energy Akv (longitudinal, -20°C ~ 20°C) ≥ 27 J.

[0058] Example 2

[0059] The method for preparing low-chromium ferritic stainless steel for highway guardrails according to Example 2 of the present invention comprises the following steps:

[0060] S1: Continuously cast slabs are produced by the process of "hot metal pretreatment + K-OBM-S converter + VOD furnace + slab continuous casting + grinding", wherein the chemical composition of the continuously cast slabs is controlled as follows by mass percentage: C 0.025%, Si 0.30%, Mn 0.90%, P 0.003%, S 0.001%, Cr 11.70%, Ni 0.20%, N 0.012%, B 0.0025%, and the balance is Fe;

[0061] S2: The continuous casting slab is heated, descaled, rough rolled, finish rolled, and coiled to produce a hot rolled coil with a thickness of 4.0 mm and a width of 1540 mm. The heating temperature of the continuous casting slab is controlled at 1150°C, and the finishing rolling temperature is controlled at 920°C.

[0062] S3: The hot rolled coil is subjected to a high-strength continuous annealing heat treatment in a continuous annealing furnace. During the high-strength continuous annealing heat treatment, the preheating temperature is controlled at 615°C, the annealing temperature is controlled at 860°C, the wire speed is controlled at 25m / min, and the cooling rate is controlled at 56°C / s.

[0063] The resulting low-chromium ferritic stainless steel for highway guardrails has the following chemical composition by mass: C 0.025%, Si 0.30%, Mn 0.90%, P 0.003%, S 0.001%, Cr 11.70%, Ni 0.20%, N 0.012%, B 0.0025%, with the balance being Fe. Metallographic examination and EBSD phase ratio analysis revealed a matrix structure of ferrite with a certain proportion of martensite. The mechanical properties of the material were tested and the results were as follows: yield strength = 385 MPa, tensile strength = 570 MPa, elongation = 30%, room temperature impact energy (sample size 4.0×10×55 mm, longitudinal) Akv = 69 J, -20°C impact energy (sample size 4.0×10×55 mm, longitudinal) Akv = 56 J, which fully meet the technical requirements for highway guardrail steel: yield strength ≥ 235 MPa, tensile strength ≥ 375 MPa, elongation ≥ 26%, impact energy Akv (longitudinal, -20°C ~ 20°C) ≥ 27 J.

[0064] Example 3

[0065] The method for preparing low-chromium ferritic stainless steel for highway guardrails according to Example 3 of the present invention comprises the following steps:

[0066] S1: Continuously cast slabs are produced by the process of "hot metal pretreatment + K-OBM-S converter + VOD furnace + slab continuous casting + grinding", wherein the chemical composition of the continuously cast slabs is controlled as follows by mass percentage: C 0.022%, Si 0.40%, Mn 0.95%, P 0.004%, S 0.001%, Cr 11.85%, Ni 0.45%, N 0.010%, B 0.0020%, and the balance is Fe;

[0067] S2: The continuous casting slab is heated, descaled, rough rolled, finish rolled, and coiled to produce a hot rolled coil with a thickness of 4.5 mm and a width of 1538 mm. The heating temperature of the continuous casting slab is controlled at 1160°C, and the finishing rolling temperature is controlled at 880°C.

[0068] S3: The hot rolled coil is subjected to a high-strength continuous annealing heat treatment in a continuous annealing furnace. During the high-strength continuous annealing heat treatment, the preheating temperature is controlled at 600°C, the annealing temperature is controlled at 875°C, the wire speed is controlled at 27m / min, and the cooling rate is controlled at 60°C / s.

[0069] The resulting low-chromium ferritic stainless steel for highway guardrails has the following chemical composition by mass: C 0.022%, Si 0.40%, Mn 0.95%, P 0.004%, S 0.001%, Cr 11.85%, Ni 0.45%, N 0.010%, B 0.0020%, with the balance being Fe. Metallographic examination and EBSD phase ratio analysis revealed a matrix structure of ferrite with a certain proportion of martensite. The mechanical properties of the material were tested as follows: yield strength = 390 MPa, tensile strength = 630 MPa, elongation = 32%, room temperature impact energy (sample size 4.5×10×55 mm, longitudinal) Akv = 88 J, -20°C impact energy (sample size 4.5×10×55 mm, longitudinal) Akv = 68 J, which fully meet the technical requirements for highway guardrail steel: yield strength ≥ 235 MPa, tensile strength ≥ 375 MPa, elongation ≥ 26%, impact energy Akv (longitudinal, -20°C ~ 20°C) ≥ 27 J.

[0070] In summary, compared with the prior art, the low-chromium ferritic stainless steel for highway guardrails and the preparation method thereof of the present invention have the following advantages and beneficial effects:

[0071] A new low-chromium ferritic stainless steel grade TGR-H for highway guardrails was obtained by optimizing the alloy composition (C≤0.030%, Si≤0.45%, Mn≤1.20%, P≤0.015%, S≤0.005%, Cr 11.50-13.50%, Ni≤0.60%, N≤0.015%, B 0.001-0.003%, and the balance was Fe). By rationally controlling the rolling process parameters (slab heating temperature at 1130-1170°C, dwell time at 200-240min, finishing rolling temperature at 880-920°C, and coiling temperature at 660-700°C), a single austenite structure was generated in the continuous casting slab at high temperature. By means of a high-strength continuous annealing heat treatment process (preheating temperature at 500-650°C, annealing temperature at 850-880°C, and line speed at 20-400°C), the slab was heat-treated to obtain a new low-chromium ferritic stainless steel grade TGR-H for highway guardrails. 0m / min, average cooling rate 50-70℃ / s), and regulating the proportion of martensite phase in the steel so that the annealed steel plate matrix becomes a complex phase structure of ferrite + a certain proportion of martensite. The material has high strength, good plasticity and toughness, and low-temperature impact toughness, and has excellent comprehensive performance. It can be used to produce highway guardrails without the need for galvanizing for corrosion protection, and fully meets the technical requirements of highway guardrail steel: yield strength ≥235MPa, tensile strength ≥375MPa, elongation ≥26%, and impact energy Akv (longitudinal, -20℃~20℃) ≥27J. Therefore, the highway guardrail produced using the low-chromium ferritic stainless steel for highway guardrails of the present invention can achieve a lightweight design of the guardrail structure while ensuring high strength and high toughness, with the beneficial effects of saving materials, energy saving and consumption reduction, cost reduction, and green environmental protection. Under the huge market scale effect of highway guardrails in the future, it will produce significant economic benefits, social benefits, and environmental benefits.

[0072] It should be noted that, in this article, the term "comprise" or any other variation thereof is intended to cover non-exclusive inclusion, so that an article or device that includes a series of elements includes not only those elements, but also includes other elements that are not explicitly listed, or also includes elements that are inherent to such article or device.

[0073] It should also be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the present invention.

Claims

1. A low chromium ferritic stainless steel for highway guardrails, characterized in that: The low-chromium ferritic stainless steel for highway guardrails has a matrix structure of ferrite + martensite, and can be used to produce highway guardrails without the need for a galvanizing anti-corrosion process. The stainless steel has a yield strength of ≥235 MPa, a tensile strength of ≥375 MPa, an elongation of ≥26%, and a longitudinal impact energy Akv of ≥27 J at -20°C to 20°C. The low-chromium ferritic stainless steel for highway guardrails is prepared by the following method: The continuous casting slab is heated, descaled, rough rolled, finish rolled, and coiled to produce hot rolled coils with a thickness of 2.0-6.0 mm. The heating temperature of the continuous casting slab is controlled at 1130-1170°C, the furnace dwell time is controlled at 200-240 minutes, the final rolling temperature is controlled at 880-920°C, and after rolling, it is put into laminar cooling. The coiling temperature is controlled at 660-700°C. The hot rolled coil is subjected to a high-strength continuous annealing heat treatment in a continuous annealing furnace. The high-strength continuous annealing heat treatment includes: the hot rolled coil is fed into the preheating zone of the annealing furnace for preheating, and the preheating temperature is controlled at 500-650°C; after preheating, the steel plate is fed into the process temperature control zone of the annealing furnace for annealing, and the annealing temperature is controlled at 850-880°C, and the wire speed is controlled at 20-40m / min; after annealing, the steel plate passes through the cooling zone of the annealing furnace and is cooled to room temperature at an average cooling rate of 50-70°C / s; Its chemical composition is controlled by mass percentage as follows: C≤0.030%, Si≤0.45%, Mn≤1.20%, P≤0.015%, S≤0.005%, Cr 11.50~13.50%, Ni≤0.60%, N≤0.015%, B 0.001~0.003%, and the balance is Fe.

2. The low chromium ferrite stainless steel for highway guardrail according to claim 1, characterized in that: The chemical composition of the low-chromium ferritic stainless steel for highway guardrails is controlled as follows by mass percentage: C 0.020%, Si 0.35%, Mn 0.85%, P 0.005%, S 0.001%, Cr 12.00%, Ni 0.22%, N 0.010%, B 0.0018%, and the balance is Fe.

3. The low chromium ferrite stainless steel for highway guardrail according to claim 1, characterized in that: The chemical composition of the low-chromium ferritic stainless steel for highway guardrails is controlled as follows by mass percentage: C 0.025%, Si 0.30%, Mn 0.90%, P 0.003%, S 0.001%, Cr 11.70%, Ni 0.20%, N 0.012%, B 0.0025%, and the balance is Fe.

4. The low chromium ferrite stainless steel for highway guardrail according to claim 1, characterized in that: The chemical composition of the low-chromium ferritic stainless steel for highway guardrails is controlled as follows by mass percentage: C 0.022%, Si 0.40%, Mn 0.95%, P 0.004%, S 0.001%, Cr 11.85%, Ni 0.45%, N 0.010%, B 0.0020%, and the balance is Fe.

5. A method for preparing low-chromium ferritic stainless steel for highway guardrails, characterized in that: It can be used to produce highway guardrails without the need for galvanizing anti-corrosion process, with yield strength ≥235MPa, tensile strength ≥375MPa, elongation ≥26%, and longitudinal -20℃~20℃ impact energy Akv ≥27J, including the following steps: S1: Continuously cast slabs are produced through the process of "hot metal pretreatment + K-OBM-S converter + VOD furnace + slab continuous casting + grinding", wherein the chemical composition of the continuously cast slabs is controlled as follows by mass percentage: C ≤ 0.030%, Si ≤ 0.45%, Mn ≤ 1.20%, P ≤ 0.015%, S ≤ 0.005%, Cr 11.50-13.50%, Ni ≤ 0.60%, N ≤ 0.015%, B 0.001-0.003%, and the balance is Fe; S2: The continuous casting slab is heated, descaled, rough rolled, finish rolled, and coiled to produce a hot rolled coil with a thickness of 2.0-6.0 mm. The heating temperature of the continuous casting slab is controlled at 1130-1170°C, the furnace dwell time is controlled at 200-240 min, the final rolling temperature is controlled at 880-920°C, and the slab is subjected to laminar cooling after rolling. The coiling temperature is controlled at 660-700°C. S3: The hot-rolled coil is subjected to a high-strength continuous annealing heat treatment in a continuous annealing furnace. The high-strength continuous annealing heat treatment includes: the hot-rolled coil is fed into the preheating zone of the annealing furnace for preheating, and the preheating temperature is controlled at 500-650°C; after preheating, the steel plate is fed into the process temperature control zone of the annealing furnace for annealing, and the annealing temperature is controlled at 850-880°C, and the wire speed is controlled at 20-40m / min; after annealing, the steel plate passes through the cooling zone of the annealing furnace and is cooled to room temperature at an average cooling rate of 50-70°C / s.

6. The method for preparing low-chromium ferrite stainless steel for highway guardrail according to claim 5, characterized in that: In step S1, the chemical composition of the continuous casting slab is controlled as follows by mass percentage: C 0.020%, Si 0.35%, Mn 0.85%, P 0.005%, S 0.001%, Cr 12.00%, Ni 0.22%, N 0.010%, B 0.0018%, and the balance is Fe; In step S2, the heating temperature of the continuous casting slab is controlled at 1145°C, and the final rolling temperature is controlled at 910°C; In step S3, the preheating temperature is controlled at 580°C, the annealing temperature is controlled at 860°C, the wire speed is controlled at 35 m / min, and the cooling rate is controlled at 62°C / s.

7. The method for preparing low-chromium ferrite stainless steel for highway guardrail according to claim 5, characterized in that: In step S1, the chemical composition of the continuous casting slab is controlled as follows by mass percentage: C 0.025%, Si 0.30%, Mn 0.90%, P 0.003%, S 0.001%, Cr 11.70%, Ni 0.20%, N 0.012%, B 0.0025%, and the balance is Fe; In step S2, the heating temperature of the continuous casting slab is controlled at 1150°C and the finishing rolling temperature is controlled at 920°C; In step S3, the preheating temperature is controlled at 615°C, the annealing temperature is controlled at 860°C, the wire speed is controlled at 25 m / min, and the cooling rate is controlled at 56°C / s.

8. The method for preparing low-chromium ferrite stainless steel for highway guardrail according to claim 5, characterized in that: In step S1, the chemical composition of the continuous casting slab is controlled as follows by mass percentage: C 0.022%, Si 0.40%, Mn 0.95%, P 0.004%, S 0.001%, Cr 11.85%, Ni 0.45%, N 0.010%, B 0.0020%, and the balance is Fe; In step S2, the heating temperature of the continuous casting slab is controlled at 1160°C and the finishing rolling temperature is controlled at 880°C; In step S3, the preheating temperature is controlled at 600°C, the annealing temperature is controlled at 875°C, the wire speed is controlled at 27 m / min, and the cooling rate is controlled at 60°C / s.

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