Production method of 700mpa grade high-strength steel for highway guardrail base

Abstract

The application discloses a steel strip for highway guardrail and a manufacturing method thereof, and belongs to the technical field of micro-alloy steel production, and the chemical components of the steel strip are as follows: [C] 0.050-0.090wt%, [Si] 0.10-0.30wt%, [Mn] 1.30-1.50wt%, [P] ≤0.018wt%, [S] ≤0.010wt%, [Als] 0.020-0.045wt%, [Nb] 0.033-0.048wt%, [Ti] 0.110-0.130wt% and [Ce] 0.003-0.01wt%. The steel strip obtained by the application has a microstructure of ferrite + pearlite, a yield strength of ≥610MPa, a tensile strength of 705-880MPa, an elongation after fracture of ≥26% and an impact energy at -20 DEG C of ≥27J (the sample size is 55mm*10mm*2.5mm), and the final performance of the product can fully meet the strict requirements of the highway guardrail in high-cold regions on the strength and toughness, bending performance and low-temperature impact resistance of the material, the production process is simple, the energy consumption is low, and the product has remarkable economy and a wide industrialization application prospect.

Description

Technical Field

[0001] This invention belongs to the field of high-strength steel production technology, specifically relating to a production method of 700MPa grade high-strength steel for highway guardrail bases. Background Technology

[0002] As a key safety facility on highways, highway guardrails absorb the kinetic energy of vehicles through plastic deformation. These guardrails require materials with high strength, high toughness, and impact resistance to improve their safety and extend their service life.

[0003] Current standards require guardrail steel to have a yield strength ≥235MPa, tensile strength ≥375MPa, elongation after fracture ≥26%, and impact energy ≥27J at -20℃ to 20℃. However, the widely used Q235 steel lacks sufficient strength and is insufficient to effectively protect against collisions with medium and heavy vehicles. With the increasing demands for protection levels, the low strength and insufficient toughness of existing carbon steel materials are becoming increasingly prominent. There is an urgent need to develop new highway guardrail materials that possess high strength and high toughness at low temperatures.

[0004] Chinese invention patent CN120967221A discloses "High-strength hot-rolled weathering steel plate for highway guardrails and its preparation method". The composition of this invention is designed as follows: 0.05%≤C≤0.08%, 0.25%≤Si≤0.35%, 1.50%≤Mn≤1.60%, P≤0.020%, S≤0.007%, 0.40≤Cr≤0.50%, 0.12≤Ni≤0.20%, 0.25≤Cu≤0.35%, 0.010≤Als≤0.045%, 0.060%≤Nb≤0.070%, 0.050%≤Ti≤0.070%, N≤0.0040%, with the remainder being iron and unavoidable impurities. This invention, through controlled smelting and rolling processes, produces a material with excellent properties such as high strength, high toughness and plasticity, high corrosion resistance, and machinability, meeting the requirements for steel used in highway guardrails. However, in terms of composition design, this invention adopts a high Mn and high Cr design, making it difficult to control core segregation; at the same time, the addition of many alloying elements results in higher smelting costs.

[0005] Chinese invention patent CN115141973A discloses "a steel strip for highway guardrails and its manufacturing method." The composition of this invention is designed as follows: 0.09%≤C≤0.16%, 0.15%≤Si≤0.5%, 1.15%≤Mn≤1.7%, P≤0.025%, S≤0.025%, 0.020%≤Al≤0.050%, 0.018%≤Nb≤0.050%, 0.015%≤Ti≤0.040%, and CEV≤0.45%. This invention uses a conventional hot continuous rolling process, and the material has high strength and toughness, meeting the requirements for steel used in highway guardrails. However, its yield strength is ≥450MPa, and its tensile strength reaches 520-680MPa, indicating room for improvement in mechanical properties. Summary of the Invention

[0006] This invention provides a method for producing 700MPa grade high-strength steel for highway guardrails, resulting in a 700MPa highway guardrail steel with a yield strength ≥610MPa, a tensile strength of 705-880MPa, and a grain size of grade 12. This solves the problem that existing guardrail steels have relatively low yield strength and tensile strength, resulting in limited protective capabilities.

[0007] Therefore, the present invention adopts the following technical solution: A 700MPa highway guardrail steel has the following chemical composition by weight percentage: [C]: 0.05-0.07wt%, [Si]: 0.10-0.30wt%, [Mn]: 1.30-1.45wt%, [P]: ≤0.018wt%, [S]: ≤0.010wt%, [Als]: 0.020-0.045wt%, [Nb]: 0.033-0.048wt%, [Ti]: 0.110-0.130wt%, [Ce]: 0.003-0.01wt%, with the remainder being iron and unavoidable trace elements.

[0008] A method for producing 700MPa grade high-strength steel for highway guardrails, comprising the following process: blast furnace hot metal → hot metal desulfurization pretreatment → converter smelting → LF refining → CSP thin slab continuous casting → roller hearth tunnel furnace homogenization → hot rolling → laminar flow cooling → coiling → inspection and warehousing; characterized in that: In the converter smelting process: the S content of the molten iron is ≤0.035wt%; the smelting process uses bottom-blowing argon gas throughout, with a bottom-blowing gas supply intensity greater than 0.02~0.05 m³ / h. 3 / (t*min); the final dissolved [O] in molten steel is controlled at 600-850ppm; deoxidation and alloying are carried out with AlFe deoxidation, and SiMnFe, SiFe, LCMnFe are used to prepare Si and Mn; CeFe is used to prepare Ce.

[0009] In the LF furnace refining process: molten steel undergoes aluminum deoxidation, Nb and V microalloying, and calcium treatment in the LF furnace, with a feed rate of [missing information]. The speed is 3-3.5 m / s, and the weak blowing time after calcium treatment is 8 min; In the CSP thin slab continuous casting process, a two-strand vertical bending CSP thin slab continuous casting process is adopted, and the casting process requires the ladle to maintain... For continuous casting, the tundish temperature is 1540~1560℃, the billet casting speed is ≥3.8m / min, carbon-free covering agent is used in the tundish, low-carbon steel protective slag is used in the crystallizer, and the liquid level fluctuation in the crystallizer is controlled within ±3mm.

[0010] In the homogenization process of the roller hearth tunnel furnace, the furnace outlet temperature is controlled at 1145-1200℃, and the billet heating temperature is optimal at 1150-1165℃. This ensures the billet is held in the furnace for a certain period of time, allowing microalloying elements to dissolve and controlling the austenite grain size. In the hot rolling process: the TMCP six-stand rolling process is adopted, focusing on controlling the reduction in the first three passes to ensure that the large reduction in rolling forms a deformation zone, increases the nucleation sites, and refines the grains; the reduction rates of F1, F2, and F3 mills are controlled at 50%, 45%, and 40% or more, respectively; at least one high-pressure water descaling is ensured during the process, with an inlet pressure of 200-260 bar, an outlet pressure of 300-380 bar, a cooling rate of less than 25℃ / s, a final rolling temperature of 880-935℃, and a coiling temperature of 560-610℃.

[0011] The role of chemical components in this invention and their impact on material properties: C: C is an important solid solution strengthening element in steel and contributes the most to its strength. Increasing the C content can significantly improve the strength of steel. However, excessively high C content is detrimental to controlling the plasticity, toughness, and formability of steel. Reducing the C content can improve plasticity and increase the toughness of steel. Therefore, to ensure the strength and formability of automotive beam steel, the C content in this invention is controlled at 0.050–0.07 wt%.

[0012] Mn: In steel, Mn partially dissolves with iron to form a solid solution (ferrite or austenite), and partially combines with iron and carbon to form cementite, which can strengthen ferrite and refine pearlite, thereby improving the strength of steel. Appropriate amounts of Mn can lower the γ→α phase transformation temperature, helping to obtain finer phase transformation products and improving strength and toughness. Simultaneously, due to the strong affinity between manganese and sulfur, it can cause sulfur in steel to form MnS, which has a higher melting point than FeS, preventing FeS precipitation at grain boundaries, reducing hot brittleness, and improving hot working performance. In this invention, the Mn content is controlled at 1.30–1.45 wt%.

[0013] Si: Si has a certain solid solution strengthening effect in steel, which is beneficial to improving the strength of steel, but not conducive to the control of toughness. However, if the Si content is too high, due to its strong diffusion ability, it will form Fe2SiO4 spinel phase with the surface iron oxide scale above 1173℃. This phase is not easy to remove during the dephosphorization process, and microcracks caused by oxide indentation are easily formed on the surface. This is detrimental to the control of the surface quality of the steel plate. In order to obtain steel plates with high strength, good formability and good surface quality, the Si content of this invention is controlled at 0.10-0.30 wt%.

[0014] Nitrogen (Nb): Nitrogen is a strong carbide and nitride-forming element. In steel, Nb exists as a substitute for solute atoms. Nb atoms are larger than Fe atoms and tend to cluster along dislocation lines, exerting a strong drag effect on dislocation climb and inhibiting recrystallization nucleation. Therefore, it can significantly delay or prevent austenite recrystallization during hot rolling before precipitation. This effect of Nb is greater than that of Ti and V. The main characteristic of Nb in steel is that it raises the recrystallization temperature of austenite, thereby refining austenite grains and achieving precipitation strengthening. Its contribution to improving the strength and toughness of the material is extremely significant. Generally, the amount of Nb added to steel is below 0.05 wt%. When the Nb content is greater than 0.05 wt%, its contribution to strengthening and toughening becomes less significant.

[0015] Ti: Ti has high solubility in steel. Taking into account the strengthening characteristics of the two elements, the 700MPa grade high-strength steel for highway guardrails of this invention adopts a niobium and titanium microalloy design: Nb content is 0.033~0.048 wt%, and Ti content is 0.110~0.130 wt%.

[0016] P and S: For most steels, P and S are harmful elements. S mainly affects the plasticity of steel, while P mainly affects the impact toughness and ductile-brittle transition temperature. In addition, sulfide inclusions in steel also have a significant impact on the properties of steel in different directions. According to the research of Japanese scholar Yukimasa Ozawa, the S content of hot-rolled low-alloy steel plates should be controlled between 0.010 and 0.015 wt%. The development trend of modern steel materials is to minimize the sulfur content in steel and control sulfide inclusions. In this invention, S is controlled below 0.010 wt%, and P is controlled below 0.018 wt%.

[0017] Ca can change the morphology of sulfides in steel and improve the plasticity and toughness of steel plates. In this invention, the Ca content is 0.0015 to 0.0040 wt%.

[0018] Als: A deoxidizing element in steel, which can reduce oxide inclusions in steel, purify steel, and improve the formability and fatigue strength of steel plates. In this invention, the Als content is 0.015 to 0.045 wt%.

[0019] Ce: The addition of rare earth cerium has the functions of purifying molten steel, improving the morphology of inclusions, refining grains, and microalloying. In this invention, the Ce content is 0.003 to 0.01 wt%.

[0020] The beneficial effects of this invention are as follows: 1. This invention adopts the design concept of low carbon, high manganese, (Nb, Ti) composite microalloyed steel. By controlling the reasonable alloy composition design, CSP production process and TMCP six-stand rolling process, it obtains steel with stable chemical composition and mechanical properties, high strength, wide cold bending performance, good plasticity and toughness, etc., which meets the requirements of steel for highway guardrails.

[0021] 2. This invention uses Nb and Ti composite microalloying to control strength. The grain size is controlled by the grain refinement and precipitation strengthening effects of Nb and Ti alloying elements, and the microstructure is further refined by recrystallization controlled rolling technology, with a grain size of 12 levels.

[0022] 3. Rare earth Ce significantly improves the toughness of V and Ti microalloyed steel by spheroidizing the long strip-shaped MnS in Nb and Ti microalloyed steel into fine and smooth rare earth inclusions, reducing element segregation, reducing banded structure and improving grain uniformity. Attached Figure Description

[0023] Figure 1 This is the metallographic diagram of Comparative Example 1; Figure 2 This is a metallographic image of Example 2; Figure 3 This is the metallographic image of Example 3. Detailed Implementation

[0024] The present invention will be further described below with reference to the accompanying drawings and specific embodiments: The following embodiments employ a CSP thin slab continuous casting and rolling process to produce 700MPa high-strength steel for highway guardrails using Nb and Ti composite microalloying technology. The process flow is as follows: blast furnace hot metal → hot metal desulfurization pretreatment → converter smelting → LF refining → CSP thin slab continuous casting → roller hearth tunnel furnace soaking → hot rolling → laminar flow cooling → coiling → inspection and warehousing; wherein: In converter smelting: a 120-ton BOF combined blowing converter is used, with the sulfur content of the molten iron entering the converter ≤ 0.035 wt%; the smelting process uses bottom blowing argon gas throughout, with a bottom blowing gas supply intensity greater than 0.02–0.05 m³ / min. 3 / (t*min); the final dissolved [O] in molten steel is controlled at 600-850ppm; AlFe is used for deoxidation and alloying, and SiMnFe, SiFe, LCMnFe are used to mix Si, Mn, and CeFe are used to mix Ce; argon is blown throughout the tapping process, the liquid surface is blown open by 300-500mm, and argon is blown after the furnace for ≥5min.

[0025] In the LF furnace refining process: After the molten steel is smelted in the converter, it undergoes aluminum deoxidation, (Nb, Ti) microalloying and calcium treatment in the LF furnace. The wire feed speed is 3-3.5 m / s, and the weak blowing time after calcium treatment is 8 min to ensure the transformation and spheroidization rate of non-metallic inclusions such as MnS and Al2O3 in the steel, so that the inclusions can float up fully and improve the cleanliness and quality of the molten steel.

[0026] In CSP thin slab continuous casting: a two-strand vertical bending CSP thin slab continuous casting process is adopted. The molten steel after converter smelting and LF furnace refining is produced by the CSP process. The casting process requires ladle protection casting and ladle slag detection and control. Low carbon steel special protective slag is used. The temperature of the tundish during continuous casting is controlled at 1540~1560℃. The billet casting speed is ≥3.8 m / min. Carbon-free covering agent is used in the tundish. Low carbon steel protective slag is used in the crystallizer. The liquid level fluctuation in the crystallizer is controlled within ±3mm.

[0027] In the homogenization process of the roller hearth tunnel furnace: the tapping temperature is controlled at 1145–1200℃, and the billet heating temperature is 1150–1165℃. This ensures sufficient holding time for the billet within the furnace, allowing microalloying elements to dissolve and controlling the austenite grain size. In hot continuous rolling: the TMCP six-stand hot continuous rolling process is adopted, focusing on controlling the reduction in the first three passes to ensure that the large reduction in rolling forms a deformation zone, increases the nucleation sites, and refines the grains; the reduction rates of F1, F2, and F3 mills are controlled at 50%, 45%, and 40% or more, respectively; at least one high-pressure water descaling is ensured during the rolling process, with a descaling inlet pressure of 200-260 bar, an outlet pressure of 300-380 bar, a cooling rate of less than 25℃ / s, a final rolling temperature of 880-935℃, and a coiling temperature of 560-610℃.

[0028] Comparative Example 1 The process is as follows: blast furnace molten iron → molten iron desulfurization pretreatment → 120-ton combined blowing converter smelting → LF refining → 2-strand vertical curved CSP thin slab continuous casting → roller hearth tunnel furnace homogenization → TMCP six-stand continuous rolling → laminar flow cooling → coiling → inspection and warehousing.

[0029] The composition of the finished product is as follows: [C]: 0.0576wt%, [Si]: 0.1550wt%, [Mn]: 1.3450wt%, [P]: 0.0138wt%, [S]: 0.0032wt%, [Nb]: 0.0391wt%, [Ti]: 0.1202wt%, [Als]: 0.0420wt%, [Ca]: 0.0021wt%, with the remainder being iron and unavoidable trace elements.

[0030] CSP thin slab continuous casting: tundish temperature 1550~1560℃, billet casting speed 3.85~3.90 m / min, carbon-free covering agent used in tundish, low-carbon steel protective slag used in crystallizer, and crystallizer liquid level fluctuation controlled within ±3mm.

[0031] Rolling process: Finished product specifications 2.5*1150 mm, furnace exit temperature 1175℃, final rolling temperature 917℃, coiling temperature 588℃, F1 reduction rate 56.3%, F2 reduction rate 48.2%, F3 reduction rate 41.3%, cooling mode adopts front-end slow cooling mode, cooling rate is 19.5m / s.

[0032] Example 2 The process is as follows: blast furnace molten iron → molten iron desulfurization pretreatment → 120-ton combined blowing converter smelting → LF refining → 2-strand vertical curved CSP thin slab continuous casting → roller hearth tunnel furnace homogenization → TMCP six-stand continuous rolling → laminar flow cooling → coiling → inspection and warehousing.

[0033] Finished product composition: [C]: 0.0543wt%, [Si]: 0.1600wt%, [Mn]: 1.3680wt%, [P]: 0.0095wt%, [S]: 0.0040wt%, [Nb]: 0.0385wt%, [Ti]: 0.1101wt%, [Als]: 0.0410wt%, [Ca]: 0.0024wt%, [Ce]: 0.0032wt%, with the remainder being iron and unavoidable trace elements.

[0034] CSP thin slab continuous casting: tundish temperature 1552~1560℃, billet casting speed 3.80~3.90 m / min, carbon-free covering agent used in tundish, low-carbon steel protective slag used in crystallizer, and crystallizer liquid level fluctuation controlled within ±3mm.

[0035] Rolling process: Finished product specifications 2.5*1150 mm, furnace exit temperature 1170℃, final rolling temperature 921℃, coiling temperature 585℃, F1 reduction rate 56.1%, F2 reduction rate 48.2%, F3 reduction rate 41.2%, cooling mode adopts front-end slow cooling mode, cooling rate is 18.1m / s.

[0036] Example 3 The process is as follows: blast furnace molten iron → molten iron desulfurization pretreatment → 120-ton combined blowing converter smelting → LF refining → 2-strand vertical curved CSP thin slab continuous casting → roller hearth tunnel furnace homogenization → TMCP six-stand continuous rolling → laminar flow cooling → coiling → inspection and warehousing.

[0037] The composition of the finished product is as follows: [C]: 0.0529wt%, [Si]: 0.1592wt%, [Mn]: 1.3526wt%, [P]: 0.0089wt%, [S]: 0.0039wt%, [Nb]: 0.0395wt%, [Ti]: 0.1204wt%, [Als]: 0.0407wt%, [Ca]: 0.0026wt%, [Ce]: 0.0036wt%, with the remainder being iron and unavoidable trace elements.

[0038] CSP thin slab continuous casting: tundish temperature 1550~1560℃, billet casting speed 3.85~3.90 m / min, carbon-free covering agent used in tundish, low-carbon steel protective slag used in crystallizer, and crystallizer liquid level fluctuation controlled within ±3mm.

[0039] Rolling process: Finished product specifications 2.5*1150 mm, furnace exit temperature 1170℃, final rolling temperature 919℃, coiling temperature 590℃, F1 reduction rate 56.2%, F2 reduction rate 48.2%, F3 reduction rate 41.4%, cooling mode adopts front-end slow cooling mode, cooling rate is 18.6m / s.

[0040] Table 1. Mechanical performance test results of embodiments of the present invention.

Claims

1. A method for producing 700MPa grade high-strength steel for highway guardrail bases, characterized in that, The chemical composition by weight percentage is as follows: [C]: 0.050~0.090wt%, [Si]: 0.10~0.30wt%, [Mn]: 1.30~1.50wt%, [P]: ≤0.018wt%, [S]: ≤0.010wt%, [Als]: 0.020~0.045wt%, [Nb]: 0.033~0.048wt%, [Ti]: 0.110~0.130wt%, [Ce]: 0.003~0.01wt%; the remainder is Fe and unavoidable trace elements. The CSP preparation method for the steel strip used in highway guardrails is as follows: blast furnace molten iron → molten iron desulfurization pretreatment → 120-ton combined blowing converter smelting → LF refining → CSP thin slab continuous casting → tunnel type soaking furnace → TMCP six-stand hot continuous rolling → laminar flow cooling → coiling → inspection and warehousing.

2. The method for producing 700MPa grade high-strength steel for highway guardrail bases according to claim 1, characterized in that, In the converter smelting process: the S content of the molten iron is ≤0.035wt%; argon gas is blown from the bottom throughout the smelting process, with a flow rate of 0.02m³. 3 / (t*min)<bottom-blowing air supply intensity≤0.05m 3 / (t*min); The final molten steel dissolution [O] is controlled at 600~850ppm. Deoxidation and alloying are carried out with AlFe deoxidation, and SiMnFe, SiFe, LCMnFe are used to prepare Si and Mn, and CeFe is used to prepare Ce.

3. The method for producing 700MPa grade high-strength steel for highway guardrail bases according to claim 1, characterized in that, In the LF refining process: molten steel undergoes aluminum deoxidation, Nb and Ti microalloying and calcium treatment in the LF furnace, with a wire feed rate of 3-3.5 m / s and a weak blowing time of 8-10 min after calcium treatment; The production method of 700MPa grade high-strength steel for highway guardrail bases according to claim 1 is characterized in that: in the CSP thin slab continuous casting: a two-strand vertical bending CSP thin slab continuous casting process is adopted; the casting process requires ladle protection casting and ladle slag detection and control; the continuous casting tundish temperature is 1540~1560℃; the billet casting speed is ≥3.8m / min; a carbon-free covering agent is used in the tundish; low-carbon steel protective slag is used in the crystallizer; and the crystallizer liquid level fluctuation is controlled within ±3mm.

4. The method for producing 700MPa grade high-strength steel for highway guardrail bases according to claim 1, characterized in that, In the tunnel-type homogenizing furnace: the furnace outlet temperature is controlled at 1145-1200℃, the billet heating temperature is 1150-1165℃, and the billet is kept at a constant temperature in the furnace to allow microalloying elements to dissolve and control the austenite grain size.

5. The method for producing 700MPa grade high-strength steel for highway guardrail bases according to claim 1, characterized in that, In the hot rolling process: the TMCP six-stand hot continuous rolling process is adopted, and the reduction of the first three passes is controlled to ensure that the large reduction of rolling forms a deformation zone, increases the nucleation position, and refines the grains; the reduction rates of F1, F2, and F3 mills are controlled at 50%, 45%, and 40% or more, respectively; at least one high-pressure water descaling is ensured during the rolling process, with a descaling inlet pressure of 200-260 bar, an outlet pressure of 300-380 bar, a cooling rate of less than 25℃ / s, a final rolling temperature of 880-935℃, and a coiling temperature of 560-610℃.

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