A low-cost Q235 grade marine atmospheric corrosion resistant steel and its manufacturing method

Abstract

This invention discloses a low-cost Q235 grade marine atmospheric corrosion resistant steel, which contains Fe and unavoidable impurity elements, and also contains the following chemical elements in the following mass percentages: C: 0.02-0.10%, 0 < Si ≤ 0.15%, Mn: 0.50-1.00%, P: 0.050-0.100%, Cu: 0.20-0.50%, Cr: 0.20-0.80%, O: 0.005-0.010%, Ce: 0.020-0.040%, 0 < Als ≤ 0.01%. Accordingly, the present invention also discloses a method for manufacturing the above-mentioned steel, which includes the following steps: (1) smelting: hot metal pretreatment, converter smelting, argon protection and LF furnace treatment; (2) continuous casting, and slow cooling of the billet after casting; (3) heating; (4) rolling: controlling the rough rolling start temperature to be not less than 980℃, the rough rolling finish temperature to be not less than 900℃, the intermediate billet thickness to be 2 to 4 times the finished product thickness, the finish rolling start temperature to be not greater than 880℃, and the finish rolling finish temperature to be 630℃ to 750℃; (5) cooling: controlling the start cooling temperature to be ≥650℃, the cooling rate to be ≤7℃ / s, and the reddening temperature to be ≤600℃.

Description

Technical Field

[0001] This invention relates to a steel plate and a method for manufacturing the same, and more particularly to a low-alloy steel and a method for manufacturing the same. Background Technology

[0002] As is well known, the development of marine resources is crucial to national energy security and defense security. The development of marine resources is of paramount importance. In the process of developing marine resources, many large-scale construction facilities are often required. Among these facilities, apart from some key structural components, most use Q235MPa grade steel. This type of steel plate, used in the marine environment, requires not only good low-temperature toughness and a low yield strength ratio, but also excellent resistance to marine atmospheric corrosion.

[0003] Currently, the manufacturing cost of existing marine atmospheric corrosion resistant steel plates is very high. Most of them contain a large amount of corrosion-resistant alloying elements, which greatly increases the overall engineering manufacturing and maintenance costs and affects their widespread use. They cannot meet the requirements of the industry's rapid development.

[0004] For example, Chinese patent document CN112126863A, published on December 25, 2020, entitled "A Weathering Steel Plate for Easy-to-Weld Bridges Resistant to Marine Climate and Its Production Method," discloses a weathering steel plate for easy-to-weld bridges resistant to marine climate and its production method. The chemical composition and mass percentage of the weathering steel plate are as follows: C: 0.06-0.10%, Mn: 1.30-1.50%, P≤0.010%, S≤0.002%, Nb: 0.015-0.035%, Ni: 1.25-1.50%, Al: 0.015-0.040%, Cu: 0.30-0.40%, Mo: 0.06-0.10%, with the remainder being Fe and unavoidable impurities. The production method includes: primary smelting, LF refining, Ca treatment, rolling, and controlled cooling processes. While this technical solution can ensure that the steel has good resistance to marine atmospheric corrosion and meets the -20℃ low temperature impact requirement, it adds a large amount of Ni alloy, has strict requirements for P content, and has a high manufacturing cost.

[0005] For example, Chinese patent document CN112647014A, published on April 13, 2021, entitled "A Structural Steel for Marine Atmospheric Environments and Its Production Method," discloses a structural steel for marine atmospheric environments and its production method. Its chemical composition and mass fraction are as follows: carbon: 0.07%–0.17%, silicon: 0.6%–0.8%, manganese: 0.3%–1.0%, phosphorus: 0.08%–0.15%, sulfur: 0.005%–0.035%, copper: 0.15%–0.2%, antimony: 0.1%–0.2%, cerium: 0.0025%–0.0045%; and selectively includes any one or two of tin: 0.01%–0.02% and vanadium: 0.05%–0.1%, with the remainder being iron and unavoidable impurities. This structural steel has excellent corrosion resistance, which can effectively improve the life and safety of steel structure buildings. However, the addition of more Si, Sb and V will increase the manufacturing cost and affect the low-temperature toughness of the steel.

[0006] For example, Chinese patent document CN111172458A, published on May 19, 2020, entitled "A Weathering Steel Resistant to High Temperature, High Humidity, and High Salt Spray Marine Atmospheric Environment and Its Preparation Method," discloses a weathering steel resistant to high temperature, high humidity, and high salt spray marine atmospheric environment and its preparation method. The chemical composition and weight percentage of the steel are: C: 0.03~0.07, Si: 0.35~0.55, Mn: 1.15~1.35, P≤0.01, S≤0.025, Cu: 0.25~0.45, Cr: 2.5~3.5, Ni: 0.9~1.1, Mo: 0.05~0.15, Sn: 0~0.6, with the remainder being Fe. In this technical solution, steel ingots conforming to the designed composition range are smelted in a vacuum induction furnace, and after subsequent controlled rolling and cooling processes, weathering steel with a bainitic microstructure is finally obtained. This weathering steel possesses excellent comprehensive properties. By increasing Cr content and reducing the high-cost Ni content, and adding microalloying elements such as Mo and Sn, it not only meets the high weather resistance requirements of the harsh service environment of the South China Sea (high temperature, high humidity, and high salt spray) but also satisfies the requirements for stable mechanical properties. However, the addition of relatively large amounts of Mn, Cr, Ni, and Mo leads to increased manufacturing costs.

[0007] Therefore, in order to meet market demand, there is an urgent need to design and develop a low-cost Q235 grade marine atmospheric corrosion resistant steel, which can effectively reduce manufacturing costs and has significant economic and environmental benefits. Summary of the Invention

[0008] One of the objectives of this invention is to provide a low-cost Q235 grade marine atmospheric corrosion resistant steel. This steel plate adopts a reasonable chemical composition design, which can achieve excellent marine atmospheric corrosion resistance while controlling low manufacturing costs. It can be used to manufacture marine platform structural components and marine environment structural components, which has great practical significance.

[0009] To achieve the above objectives, the present invention provides a low-cost Q235 grade marine atmospheric corrosion resistant steel, which contains Fe and unavoidable impurity elements, and also contains the following chemical elements in the following mass percentages:

[0010] C: 0.02~0.10%, 0<Si≤0.15%, Mn: 0.50~1.00%, P: 0.050~0.100%, Cu: 0.20~0 .50%, Cr: 0.20~0.80%, O: 0.005~0.010%, Ce: 0.020~0.040%, 0<Als≤0.01%.

[0011] Furthermore, in the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention, the mass percentage content of each chemical element is as follows:

[0012] C: 0.02–0.10%, 0 < Si ≤ 0.15%, Mn: 0.50–1.00%, P: 0.050–0.100%, Cu: 0.20–0.50%, Cr: 0.20–0.80%, O: 0.005–0.010%, Ce: 0.020–0.040%, 0 < Als ≤ 0.01%; balance is Fe and unavoidable impurity elements.

[0013] In designing the invention, the inventors considered that the steel used in this invention is primarily intended for use in marine environments, where excellent strength, toughness, and resistance to seawater corrosion are desirable. Therefore, during actual steelmaking, it is necessary to precisely control the oxygen content of the molten steel while removing large inclusions generated by high oxygen content, thereby reducing defects such as porosity on the surface of the cast billet during continuous casting.

[0014] Based on this consideration, the inventors rationally designed the content of C and Mn elements in the chemical composition design to ensure the basic strength of the steel, while strictly controlling the content of P, Cu, Cr, O and Ce elements to improve the steel's resistance to seawater corrosion.

[0015] The design principles of each chemical element in the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention are as follows:

[0016] C: In the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention, carbon (C) is the most effective element for improving the strength of the steel. As the C content in the steel increases, the Fe3C content also increases, which in turn increases the hardenability of the steel, thereby improving its yield strength and tensile strength. However, it should be noted that during the smelting process, C in the steel reacts with oxygen (O), and a C / O balance exists. Therefore, to ensure the O content in the steel and to ensure the performance of the steel, the mass percentage of C in this invention is controlled between 0.02% and 0.10%.

[0017] Si: In the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention, Si has a weak affinity for C and does not combine with C in the steel. However, Si can dissolve in ferrite and produce solid solution strengthening, which increases the strength and hardness of the ferrite. This, however, also leads to a decrease in the plasticity and toughness of the steel. Although adding Si to steel helps to improve the material's strength, Si, as a deoxidizer, has a stronger deoxidizing ability than Mn. Therefore, the amount of Si added to the steel should not be too high. When the Si content in the steel increases, the O content in the steel decreases. Therefore, in the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention, the mass percentage of Si is controlled to be 0 < Si ≤ 0.15%.

[0018] Mn: In the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention, Mn has a strong affinity for C. It is an effective element for expanding the austenite phase region, refining grains, ensuring the comprehensive performance of the steel, and improving hardenability, without deteriorating the steel's deformability. It should be noted that adding 1% Mn to steel can increase the tensile strength by approximately 100 MPa. However, Mn is an element prone to segregation. When the Mn and C content in the segregation region reaches a certain proportion, martensite phase will be generated during steel production and welding. This phase exhibits very high hardness and has a significant impact on the welding performance of equipment. Therefore, the Mn content is limited to no more than 1.00% when designing this steel. Thus, considering the strength range of the steel, the mass percentage of Mn in the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention is controlled between 0.50% and 1.00%.

[0019] P: In the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention, phosphorus (P) has a strong solid solution strengthening and cold work hardening effect. It can be added to the steel as an alloying element, improving its strength and atmospheric corrosion resistance, but it also reduces its cold stamping performance. In practical use, P in the steel forms phosphate ions during marine atmospheric corrosion. can and Fe 3+The reaction generates an insoluble γ-Fe₂O₄ and FePO₄·H₂O mixture film, hindering metal ionization and thus slowing down metal corrosion. Meanwhile, phosphorus in steel mainly exists in solid solution and as phosphides (Fe₂P, Fe₃P, etc.). Although it can improve the strength and hardness of steel, its biggest drawback is severe segregation. Excessive P content in steel will significantly reduce its plasticity and toughness. Therefore, considering all factors, in the low-cost Q235 marine atmospheric corrosion-resistant steel described in this invention, the mass percentage of P is strictly controlled between 0.050% and 0.100%.

[0020] Cu: In the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention, Cu mainly plays a precipitation strengthening role, which is beneficial to the atmospheric corrosion resistance of the steel. Adding an appropriate amount of Cu to the steel can not only improve the atmospheric corrosion resistance of the steel, but also improve the fatigue crack propagation resistance of the steel. At the same time, Cu can also reduce the deterioration effect of sulfur content on the corrosion resistance of the steel plate, and can reduce the deterioration of the corrosion resistance of the steel plate under both high and low sulfur conditions. However, it should be noted that the Cu content in the steel should not be too high. When the Cu content in the steel is too high, the steel is prone to network cracks during rolling. Based on this, considering the comprehensive influence of Cu on the mechanical properties of the steel plate, in the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention, the mass percentage of Cu is controlled between 0.20% and 0.50%.

[0021] Cr: In the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention, Cr is a commonly used element in corrosion-resistant steel. During the corrosion process, compared with ordinary carbon steel and low-alloy steel, the corrosion film formed by this steel is enriched with Cr. The enrichment of Cr can improve the stability of the corrosion film and effectively delay corrosion deterioration. Based on this, considering alloy cost and usage requirements, in this invention, the mass percentage of Cr is controlled between 0.20% and 0.80%.

[0022] O: In the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention, the content of oxygen (O) in the steel must be strictly controlled. Excessive O content easily generates more oxygen-containing inclusions, causing surface defects such as porosity during continuous casting. However, it should be noted that the presence of a certain amount of O in the steel can shorten the length of inclusions, making them ellipsoidal or granular, and reducing stress concentration at the inclusion tips, thus lowering the risk of pitting corrosion. Simultaneously, a high dissolved O content in the steel also improves the thermodynamic stability of the matrix, which is beneficial for enhancing the corrosion resistance of the steel plate. Based on this, considering the comprehensive influence of O content on the steel properties, the mass percentage of O in the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention is strictly controlled between 0.005% and 0.010%.

[0023] Ce: In the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention, Ce exists in the steel in the form of Ce2O2S, Ce3S4, and CeS. It can form non-metallic inclusions with Al2O3, TiO, MnO, and SiO2 as the core and Ce2O2S, Ce2S3, or CeS as the surface. The corrosion products formed during the corrosion process can inhibit the further development of corrosion. However, the Ce content in the steel should not be too high. If the rare earth Ce content in the steel is too high, rare earth inclusions will easily aggregate, which is not conducive to the low-temperature toughness of the steel plate. Based on this, in this invention, the mass percentage of Ce element is controlled between 0.020% and 0.040%.

[0024] Al: In the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention, the maximum solubility of Al in austenite is approximately 0.6%. After dissolving into austenite, it only slightly increases the hardenability of the material. During design, the Al content in the steel should not be too high. When the Al content is too high, it easily leads to an increase in inclusions in the steel, adversely affecting the toughness of the steel, reducing its hardenability and toughness, and increasing the banded structure level. Simultaneously, Al is also a strong deoxidizing element in steel; increasing the Al content will significantly reduce the O content. Therefore, in the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention, the Al content must be strictly controlled, keeping it at 0 < Als ≤ 0.01%.

[0025] Furthermore, in the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention, S ≤ 0.005% among unavoidable impurity elements.

[0026] In the above technical solution, S is an impurity element in the low-cost Q235 grade marine atmospheric corrosion resistant steel of the present invention. Under the condition that the technical conditions permit, in order to obtain steel with better performance and better quality, the content of impurity elements in the steel should be reduced as much as possible.

[0027] In this invention, sulfur (S) is detrimental to the stress corrosion cracking stability of steel; as the S content in the steel increases, the stability of the steel deteriorates sharply. Furthermore, sulfide inclusions are hydrogen accumulation sites, leading to defective structures in the metal. Additionally, sulfur is a promoter of hydrogen adsorption, which is also detrimental to the performance of the steel. Therefore, in the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention, the mass percentage of sulfur is controlled to meet the requirement: S ≤ 0.005%.

[0028] Furthermore, in the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention, each chemical element satisfies at least one of the following formulas:

[0029] 10≤Si / O≤15;

[0030] 5≤Mn / Si≤15;

[0031] Ceq≤0.33, where Ceq=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14, and each chemical element in the formula is replaced with the value before the percentage sign of its mass percentage content.

[0032] In the above-described technical solution of this invention, while controlling the mass percentage content of a single chemical element in steel, this invention can also control the Si and O elements to satisfy the limiting relationship of 10 ≤ Si / O ≤ 15, wherein the values ​​of the aforementioned elements are all substituted with the values ​​before the percentage sign of their mass percentage content. This limiting relationship can ensure the effect of weak deoxidation of Si during the steel smelting process.

[0033] Accordingly, the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention can control the mass percentage content of a single chemical element in the steel, while also controlling the Mn and Si elements to meet the constraint relationship of 5 ≤ Mn / Si ≤ 15, where each chemical element is substituted with the value before the percentage sign of its mass percentage content. This constraint relationship can ensure the range of Si and Mn content during the steel smelting process, so as to guarantee the weak deoxidation effect and the contribution of Si and Mn to the strength of the steel plate.

[0034] Furthermore, in the design of chemical composition, the present invention can further control Ceq≤0.33 to ensure the weldability of the steel plate and reduce the tendency of welding cracks.

[0035] Furthermore, in the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention, its microstructure is ferrite + pearlite, wherein the proportion of ferrite is more than 70%.

[0036] Furthermore, in the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention, it has composite inclusions, the center of which is Al2O3, TiO, MnO and SiO2, and the surface is Ce2O2S, Ce2S3 or CeS.

[0037] Furthermore, in the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention, the following properties are characterized: yield strength at room temperature ≥235MPa, tensile strength 370~500MPa, elongation ≥26%; impact energy KV2 at -20℃ ≥80J; hardness ≤160HBW; and corrosion rate in accelerated immersion corrosion test ≤1.3g / (m²). 2 ·h).

[0038] Furthermore, in the low-cost Q235 grade marine atmospheric corrosion resistant steel described in this invention, its yield strength at room temperature is ≥300MPa.

[0039] Accordingly, another objective of the present invention is to provide a method for manufacturing the aforementioned low-cost Q235 grade marine atmospheric corrosion resistant steel. This manufacturing method has optimized the process design, resulting in lower manufacturing costs. The Q235 grade marine atmospheric corrosion resistant steel obtained by this manufacturing method not only has excellent mechanical properties but also good resistance to marine atmospheric corrosion.

[0040] To achieve the above objectives, this invention proposes a method for manufacturing the aforementioned low-cost Q235 grade marine atmospheric corrosion resistant steel, comprising the following steps:

[0041] (1) Smelting: hot metal pretreatment, converter smelting, argon protection and LF furnace treatment;

[0042] (2) Continuous casting, after pouring, the billets are stacked and cooled slowly;

[0043] (3) Heating;

[0044] (4) Rolling: The roughing rolling start temperature is not less than 980℃, the roughing rolling finish temperature is not less than 900℃, the intermediate billet thickness is 2 to 4 times the finished product thickness, the finishing rolling start temperature is not greater than 880℃, and the finishing rolling finish temperature is 630℃ to 750℃.

[0045] (5) Cooling: Control the starting temperature of cooling to ≥650℃, the cooling rate to ≤7℃ / s, and the temperature at which the temperature returns to red to ≤600℃.

[0046] In the above-described technical solution of this invention, based on the optimized chemical composition, a reasonable manufacturing method is further optimized. In this manufacturing method, a controlled rolling and cooling (TMCP) process is adopted for the heated billet, and the process parameters of the controlled rolling and cooling (TMCP) process are strictly controlled to obtain a stable ferrite + pearlite structure, thereby ensuring that the steel obtains good strength, toughness, and resistance to marine atmospheric corrosion.

[0047] The controlled cooling process (TMCP) of this invention specifically includes the rolling cooling processes in steps (4) and (5) above. During rough rolling, the thickness of the intermediate billet at the end of this stage can be precisely controlled according to the thickness of the finished steel plate. During finish rolling, controlled rolling of the non-recrystallized austenite region only begins after the temperature has been avoided from the recrystallization zone of austenite. At this point, there is a sufficient compression ratio for rolling the non-recrystallized austenite region, allowing for the accumulation of highly distorted deformation in the deformed austenite, forming numerous deformation bands and high-density dislocations. After the final finish rolling is completed, the deformed dislocations will recover and become polygonal, thereby refining the microstructure and improving the strength and toughness of the steel plate.

[0048] This rolling process can ensure that the strength and toughness of the steel plate meet the design requirements under low carbon equivalent conditions. Therefore, taking all factors into consideration, in the rolling process of step (4) of this invention, the roughing rolling start temperature is controlled to be no less than 980℃, the roughing rolling finish temperature is no less than 900℃, the finishing rolling start temperature is no greater than 880℃, and the finishing rolling finish temperature is 630℃~750℃. At the same time, in order to ensure that the grains are sufficiently reduced, the thickness of the intermediate billet is set to 2 to 4 times the thickness of the finished product.

[0049] Accordingly, based on the strength grade requirements of the steel grade, under the rolling deformation conditions set in the process, the hardness of the steel grade increases with the increase of the cooling rate. Therefore, in this invention, with the statistical average hardness of the steel plate designed as 150 HBW as the target hardness value, the cooling rate to obtain the ideal hardness level of the steel should be less than or equal to 7℃ / s. Under this condition, the microstructure of the steel plate is ferrite + pearlite, with the ferrite content accounting for more than 70%. Therefore, in the cooling step of step (5) above, the cooling rate is controlled to be ≤7℃ / s.

[0050] In addition, it should be noted that in the smelting process of step (1) of the present invention, iron desulfurization technology can be used for iron pretreatment, and then a converter can be used for smelting, top and bottom blowing in the converter, and Si-Mn deoxidation treatment in the converter, thereby effectively changing the shape of the inclusions from long strips to short strips and spherical inclusions, reducing the stress zone at the tip of the inclusions, reducing the corrosion tendency, and improving the pitting corrosion resistance of the steel.

[0051] Meanwhile, after the converter smelting is completed, argon protection and LF furnace treatment are required. During the LF heating furnace treatment, the composition can be finely adjusted according to the loss of alloying elements under high oxygen conditions. However, it is necessary to avoid prolonged disturbance of the molten steel to prevent the oxygen from being consumed by oxidation in the molten steel, which would cause the oxygen content in the steel to fail to reach the target value.

[0052] Accordingly, in step (2) above, the molten steel after smelting needs to be continuously cast to obtain a billet. During the continuous casting crystallizer process, rare earth wire can be added to modify inclusions, and the billet is stacked and cooled slowly after casting.

[0053] In addition, ultra-low carbon protective slag can be used during continuous casting to prevent the carbon in the protective slag from reacting with the oxygen in the molten steel, causing defects such as bubbles to form on the inner surface of the billet. In actual continuous casting, a vertical continuous casting machine can be used to facilitate the flotation of large inclusions in the steel and reduce segregation. After the continuous casting process is completed, the surface of the billet can be cleaned to remove surface porosity and other defects.

[0054] Furthermore, in the manufacturing method described in this invention, in step (3), the heating temperature is controlled to be 1000-1100℃ and the heating rate is controlled to be 8-15 min / cm.

[0055] In the above technical solution of the present invention, since the alloy content of the steel of the present invention is very low, the billet can preferably be heated at low temperature during the heating process in step (3), and the heating temperature is controlled at 1000 to 1100°C and the heating rate is controlled at 8 to 15 min / cm, so as to reduce the heating energy consumption and reduce the manufacturing cost while ensuring the uniform temperature of the billet.

[0056] Furthermore, in the manufacturing method described in this invention, in step (4), the reduction rate of the roughing pass is controlled to be ≥12%, the reduction rate of the finishing pass is controlled to be ≥10%, the reduction rate of the last three finishing passes is controlled to be ≥30%, and the total reduction rate of the finishing is controlled to be ≥50%.

[0057] Furthermore, in the manufacturing method described in this invention, for steel plates with a finished thickness of 6 to 16 mm, in step (4), the roughing rolling start temperature is controlled to be no less than 980°C, the roughing rolling finish temperature is controlled to be no less than 900°C, the intermediate billet thickness is 20 to 40 mm, the finishing rolling start temperature is no greater than 880°C, the finishing rolling passes are 6 to 7, and the finishing rolling finish temperature is 700°C to 750°C; in step (5), the cooling start temperature is controlled to be ≥650°C, the cooling rate is 4 to 6°C / s, and the reddening temperature is ≤600°C.

[0058] Furthermore, in the manufacturing method described in this invention, for steel plates with a finished thickness of 16 mm to 30 mm, in step (4), the roughing rolling start temperature is controlled to be no less than 980°C, the roughing rolling finish temperature is controlled to be no less than 900°C, the intermediate billet thickness is 40 to 60 mm, the finishing rolling start temperature is no greater than 850°C, the finishing rolling passes are 5 to 6, and the finishing rolling finish temperature is 630°C to 700°C; in step (5), the cooling start temperature is controlled to be ≥680°C, the cooling rate is 5 to 7°C / s, and the reddening temperature is ≤580°C.

[0059] The low-cost Q235 grade marine atmospheric corrosion resistant steel and its manufacturing method described in this invention have the following advantages and beneficial effects compared with the prior art:

[0060] Existing marine atmospheric corrosion resistant steel plates generally have high corrosion-resistant alloy content, resulting in high manufacturing costs and overall high engineering costs and maintenance, which cannot meet the requirements of rapid industry development. Therefore, to overcome the shortcomings of existing technologies, this invention adopts a reasonable chemical composition design and, in conjunction with a feasible production process, designs and develops a low-cost Q235 grade marine atmospheric corrosion resistant steel. This steel is suitable for large-scale production operations, meets market demands, reduces manufacturing costs, and has significant economic and environmental benefits.

[0061] In designing the chemical element composition, this invention employs a low-C, low-Si, low-Mn, low-Al, high-P, and high-O design, while controlling the addition of certain amounts of Cu, Cr, and Ce elements, and strictly controlling the content of the impurity element S, to ensure that the steel achieves excellent resistance to marine atmospheric corrosion. Correspondingly, in the manufacturing process design, the inventors rationally adopted a controlled rolling and cooling (TMCP) process to ensure that the steel achieves good strength and toughness, resulting in a stable ferrite + pearlite microstructure.

[0062] Based on this design, the steel ultimately prepared by this invention possesses good strength and toughness, as well as excellent resistance to marine atmospheric corrosion. Its yield strength at room temperature is ≥235 MPa, tensile strength is 370–500 MPa, elongation is ≥26%, impact energy at -20℃ (KV2) is ≥80 J, hardness is ≤160 HBW, and corrosion rate in accelerated immersion corrosion tests is ≤1.3 g / (m²). 2 ·h).

[0063] Therefore, it is evident that the steel of this invention, through compositional design, inclusion control, rolling, and cooling, can achieve excellent resistance to marine atmospheric corrosion while maintaining low manufacturing costs. The production process of this low-cost Q235 grade marine atmospheric corrosion resistant steel is simple and feasible, suitable for large-scale production operations. It can be used to manufacture various large marine platform structural components and marine atmospheric environment building structural components, meeting market demands and possessing significant practical value. Attached Figure Description

[0064] Figure 1 The image shows the metallographic structure of the low-cost Q235 grade marine atmospheric corrosion resistant steel of Example 3 under a 200x optical electron microscope. Detailed Implementation

[0065] The following will further explain and illustrate the low-cost Q235 grade marine atmospheric corrosion resistant steel and its manufacturing method according to the present invention with reference to specific embodiments and accompanying drawings. However, this explanation and illustration do not constitute an undue limitation on the technical solution of the present invention.

[0066] Examples 1-6 and Comparative Examples 1-2

[0067] The low-cost Q235 grade marine atmospheric corrosion resistant steels described in Examples 1-6 of this invention are all prepared using the following steps:

[0068] (1) Smelting is carried out according to the chemical composition shown in Table 1-1 and Table 1-2: hot metal desulfurization pretreatment, converter smelting, argon protection and LF furnace treatment are carried out in sequence.

[0069] (2) Continuous casting: A vertical continuous casting machine is used for continuous casting. Rare earth wire is added during continuous casting to modify inclusions. After casting, the billets are stacked for slow cooling. At the same time, ultra-low carbon protective slag is used during continuous casting to prevent the carbon in the protective slag from reacting with the oxygen in the molten steel.

[0070] (3) Heating: Control the heating temperature to 1000~1100℃ and the heating rate to 8~15min / cm.

[0071] (4) Rolling: The roughing rolling start temperature shall be no less than 980℃, the roughing rolling finish temperature shall be no less than 900℃, the intermediate billet thickness shall be 2 to 4 times the finished product thickness, the finishing rolling start temperature shall be no greater than 880℃, the finishing rolling finish temperature shall be 630℃ to 750℃, and the roughing rolling pass reduction rate shall be ≥12%, the finishing rolling pass reduction rate shall be ≥10%, the finishing rolling last three passes reduction rate shall be ≥30%, and the finishing rolling total reduction rate shall be ≥50%.

[0072] For steel plates with a finished thickness of 6 to 16 mm, it is preferable to control the roughing rolling start temperature to be no less than 980℃, the roughing rolling finish temperature to be no less than 900℃, the intermediate billet thickness to be 20 to 40 mm, the finishing rolling start temperature to be no more than 880℃, the finishing rolling passes to be 6 to 7, and the finishing rolling finish temperature to be 700℃ to 750℃.

[0073] For steel plates with a finished thickness of 16mm to 30mm, it is preferable to control the roughing rolling start temperature to be no less than 980℃, the roughing rolling finish temperature to be no less than 900℃, the intermediate billet thickness to be 40-60mm, the finishing rolling start temperature to be no more than 850℃, the finishing rolling passes to be 5-6 times, and the finishing rolling finish temperature to be 630℃-700℃.

[0074] (5) Cooling: Control the starting cooling temperature ≥650℃, cooling rate ≤7℃ / s, and reddening temperature ≤600℃. Specifically, for steel plates with a finished thickness of 6~16mm, it is preferable to control the starting cooling temperature ≥650℃, and specifically control the water flow rate in the weak cooling zone to 40L / s, the water flow rate in the weak cooling zone to 100L / s, the water flow rate in the strong cooling zone to 100L / s, the water flow rate in the strong cooling zone to 350L / s, the roller speed to 0.8m / s, the cooling rate to 4~6℃ / s, and the reddening temperature ≤600℃. For steel plates with a finished thickness of 16mm to 30mm, it is preferable to control the initial cooling temperature to ≥680℃, and specifically control the water flow rate in the weak cooling zone to 50L / s, the water flow rate in the weak cooling zone to 100L / s, the water flow rate in the strong cooling zone to 120L / s, the water flow rate in the strong cooling zone to 350L / s, the roller speed to 0.7m / s, the cooling rate to 5~7℃ / s, and the reddening temperature to ≤580℃.

[0075] It should be noted that, in actual experiments, after completing the above steps (1)-(5), the low-cost Q235 grade marine atmospheric corrosion resistant steels obtained in Examples 1-6 can be subjected to further flaw detection, finishing, and inspection. In this invention, the chemical composition design and related processes of the low-cost Q235 grade marine atmospheric corrosion resistant steels in Examples 1-6 all meet the design specifications of this invention.

[0076] Accordingly, in order to demonstrate the superiority of the present invention, the inventors further set up two comparative examples, namely Comparative Examples 1-2. Unlike the controlled rolling and cooling (TMCP) process used in Examples 1-6, the comparative steel plates of Comparative Examples 1-2 were prepared by hot rolling. Both of these comparative steel plates are existing conventional Q235B structural steels in the prior art. Moreover, Comparative Examples 1-2 have parameters in their chemical composition design and related processes that do not meet the design requirements of the present invention.

[0077] Tables 1-1 and 1-2 list the mass percentages of each chemical element in the low-cost Q235 grade marine atmospheric corrosion resistant steel of Examples 1-6 and the comparative steel plates of Comparative Examples 1-2.

[0078] Table 1-1. (wt%, balance Fe and other unavoidable impurities besides S)

[0079]

[0080] Table 1-2.

[0081]

[0082]

[0083] Note: In the table above, Ceq=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14, where each chemical element is represented by the value before the percentage sign of its mass percentage content.

[0084] Tables 2-1 and 2-2 list the specific process parameters for the low-cost Q235 grade marine atmospheric corrosion resistant steel of Examples 1-6 and the comparative steel plates of Comparative Examples 1-2.

[0085] Table 2-1.

[0086]

[0087] Table 2-2.

[0088]

[0089] Samples were taken from the low-cost Q235 grade marine atmospheric corrosion resistant steel of Examples 1-6 and the comparative steel plates of Comparative Examples 1-2 obtained through the above process steps. The steels of each example and comparative example were observed and analyzed. It was observed that the microstructure of the low-cost Q235 grade marine atmospheric corrosion resistant steel of Examples 1-6 was ferrite + pearlite, and the proportion of ferrite was more than 70%.

[0090] In addition, composite inclusions were found in the low-cost Q235 grade marine atmospheric corrosion resistant steel of Examples 1-6 and the comparative steel plates of Comparative Examples 1-2. The inventors tested the composite inclusions in each example and comparative example and listed the results of the inclusion test in Table 3 below.

[0091] Table 3 lists the inclusion detection results of the low-cost Q235 grade marine atmospheric corrosion resistant steel of Examples 1-6 and the control steel plates of Comparative Examples 1-2.

[0092] Table 3.

[0093]

[0094] As shown in Table 3, an observation of the composite inclusions in the low-cost Q235 grade marine atmospheric corrosion resistant steel of Examples 1-6 and the comparative steel plates of Comparative Examples 1-2 reveals that, compared to Comparative Examples 1-2, the inclusions in Examples 1-6 are of lower grade and smaller size, thus improving the corrosion resistance of the steel plates.

[0095] Observation of inclusions in the low-cost Q235 grade marine atmospheric corrosion resistant steel of Examples 1-6 shows that the core of the composite inclusions in the low-cost Q235 grade marine atmospheric corrosion resistant steel of Examples 1-6 is Al2O3, TiO, MnO and SiO2, and the surface is Ce2O2S, Ce2S3 or CeS. This composite inclusion has good pitting corrosion resistance.

[0096] Accordingly, after completing the above observation of the microstructure, it is necessary to further test the mechanical properties of the steel in each embodiment and comparative example to obtain the performance of the steel in each embodiment and comparative example. The relevant mechanical property test results are listed in Table 4.

[0097] The relevant performance testing methods are as follows:

[0098] (1) Tensile test: The tensile test was conducted at room temperature in accordance with GB / T228.1 "Metallic materials - Tensile testing - Part 1: Test method at room temperature" to obtain the yield strength, tensile strength and elongation values ​​of the steels in each example and comparative example.

[0099] (2) Impact test: The impact test was conducted in accordance with GB / T 229 "Metallic materials Charpy pendulum impact test method" to test the transverse impact performance KV2 value of the steel in each example and comparative example at -20℃.

[0100] (3) Hardness test: The hardness test was carried out in accordance with GB / T 231.1 "Metallic materials Brinell hardness test - Part 1: Test method" to obtain the hardness of the steel in each example and comparative example.

[0101] (4) Marine atmospheric corrosion resistance test: The corrosion test was carried out in accordance with TB / T 2375 "Railway weathering steel cyclic immersion corrosion test method" to obtain the corrosion rate of the steel in each example and comparative example in the cyclic immersion accelerated corrosion test.

[0102] Table 4 lists the observation results and mechanical property test results of the low-cost Q235 grade marine atmospheric corrosion resistant steel of Examples 1-6 and the comparative steel plates of Comparative Examples 1-2.

[0103] Table 4.

[0104]

[0105] As shown in Table 4, compared with the comparative steel plates of Examples 1-2, the low-cost Q235 grade marine atmospheric corrosion resistant steels of Examples 1-6 exhibit significantly superior comprehensive performance. The low-cost Q235 grade marine atmospheric corrosion resistant steels of Examples 1-9 of this invention have a yield strength of 310-350 MPa, a tensile strength of 420-460 MPa, an elongation of 35-38% at room temperature, an impact energy (KV2) of 80-115 J at -20℃, a hardness of 140-160 HBW, and a corrosion rate of 1.20-1.30 g / (m²) in accelerated immersion corrosion tests. 2 Between ·h).

[0106] Compared with the conventional Q235B structural steel of Comparative Examples 1 and 2, the steel plates of Examples 1-6 of the present invention have improved strength, plasticity and toughness at room temperature, and their corrosion resistance in marine atmospheric environments is 2 to 3 times that of conventional Q235B.

[0107] It is evident that the steel designed and prepared using this invention can achieve significant improvements in both resistance to marine atmospheric corrosion and safety. It can be used to manufacture various large marine platform structural components and marine atmospheric environment building structural components, meeting market demands and possessing significant practical value.

[0108] Figure 1 The image shows the metallographic structure of the low-cost Q235 grade marine atmospheric corrosion resistant steel of Example 3 under a 200x optical electron microscope.

[0109] like Figure 1 As shown, in this embodiment, the microstructure of the low-cost Q235 grade marine atmospheric corrosion resistant steel of Example 3 is ferrite + pearlite, with the ferrite phase ratio exceeding 70%. It should be noted that the steel also contains composite inclusions, the core of which is Al2O3, TiO, MnO, and SiO2, and the surface is Ce2O2S, Ce2S3, or CeS.

[0110] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0111] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A low-cost Q235 grade marine atmospheric corrosion resistant steel, characterized in that, Its mass percentage content of each chemical element is as follows: C: 0.02~0.10%, 0<Si≤0.15%, Mn: 0.50~1.00%, P: 0.050~0.100%, Cu: 0.20~0.50%, Cr: 0.20~0.80%, O: 0.005~0.010%, Ce: 0.020~0.040%, 0<Als≤0.01%, balance being Fe and unavoidable impurity elements; The low-cost Q235 grade marine atmospheric corrosion resistant steel has composite inclusions, the core of which is Al2O3, TiO, MnO and SiO2, and the surface is Ce2O2S, Ce2S3 or CeS.

2. The low-cost Q235 grade marine atmospheric corrosion resistant steel as described in claim 1, characterized in that, In unavoidable impurity elements, S ≤ 0.005%.

3. The low-cost Q235 grade marine atmospheric corrosion resistant steel as described in claim 1, characterized in that, Each chemical element satisfies at least one of the following formulas: 10≤Si / O≤15; 5≤Mn / Si≤15; Ceq≤0.33, where Ceq=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14, and each chemical element in the formula is replaced with the value before the percentage sign of its mass percentage content.

4. The low-cost Q235 grade marine atmospheric corrosion resistant steel as described in claim 1, characterized in that, Its microstructure consists of ferrite and pearlite, with ferrite accounting for more than 70%.

5. The low-cost Q235 grade marine atmospheric corrosion resistant steel as described in claim 1, characterized in that, Its performance meets the following requirements: yield strength at room temperature ≥235MPa, tensile strength 370~500MPa, elongation ≥26%; impact energy KV2 ≥80J at -20℃; hardness ≤160HBW; and corrosion rate ≤1.3g / (m²) in accelerated immersion corrosion test. 2 ·h).

6. The low-cost Q235 grade marine atmospheric corrosion resistant steel as described in claim 5, characterized in that, Its yield strength at room temperature is ≥300MPa.

7. The method for manufacturing low-cost Q235 grade marine atmospheric corrosion resistant steel as described in any one of claims 1-6, characterized in that, It includes the following steps: (1) Smelting: hot metal pretreatment, converter smelting, argon protection and LF furnace treatment; (2) Continuous casting, followed by slow cooling of the billet by stacking; (3) Heating; (4) Rolling: The roughing rolling start temperature is not less than 980℃, the roughing rolling finish temperature is not less than 900℃, the intermediate billet thickness is 2 to 4 times the finished product thickness, the finishing rolling start temperature is not greater than 880℃, and the finishing rolling finish temperature is 630℃ to 750℃. (5) Cooling: Control the starting temperature of cooling to ≥650℃, the cooling rate to ≤7℃ / s, and the red temperature to ≤600℃.

8. The manufacturing method as described in claim 7, characterized in that, In step (3), the heating temperature is controlled at 1000~1100℃ and the heating rate is controlled at 8~15min / cm.

9. The manufacturing method as described in claim 7, characterized in that, In step (4), the reduction rate of roughing passes is controlled to be ≥12%, the reduction rate of finishing passes is controlled to be ≥10%, the reduction rate of the last three finishing passes is controlled to be ≥30%, and the total reduction rate of finishing is controlled to be ≥50%.

10. The manufacturing method as described in claim 7, characterized in that, For steel plates with a finished thickness of 6 to 16 mm, in step (4), the roughing rolling start temperature is controlled to be no less than 980℃, the roughing rolling finish temperature is controlled to be no less than 900℃, the intermediate billet thickness is 20 to 40 mm, the finishing rolling start temperature is no greater than 880℃, the finishing rolling passes are 6 to 7, and the finishing rolling finish temperature is 700℃ to 750℃; in step (5), the cooling start temperature is controlled to be ≥650℃, the cooling rate is 4 to 6℃ / s, and the reddening temperature is ≤600℃.

11. The manufacturing method as described in claim 7, characterized in that, For steel plates with a finished thickness of 16mm to 30mm, in step (4), the roughing rolling start temperature is controlled to be no less than 980℃, the roughing rolling finish temperature is controlled to be no less than 900℃, the intermediate billet thickness is 40~60mm, the finishing rolling start temperature is no greater than 850℃, the finishing rolling passes are 5~6 times, and the finishing rolling finish temperature is 630℃~700℃; in step (5), the cooling start temperature is controlled to be ≥680℃, the cooling rate is 5~7℃ / s, and the reddening temperature is ≤580℃.

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