A weakly deoxidized Q355 grade seawater corrosion resistant steel and its manufacturing method

Abstract

This invention discloses a weakly deoxidized Q355 grade seawater corrosion resistant steel, which contains Fe and unavoidable impurity elements, and also contains the following chemical elements in the following mass percentages: C: 0.05-0.15%, 0 < Si ≤ 0.15%, Mn: 0.80-1.50%, P: 0.030-0.080%, Cr: 0.20-0.80%, Nb: 0.010-0.050%, Ti: 0.010-0.020%, O: 0.010-0.025%, Ce: 0.020-0.040%, 0 < Als ≤ 0.010%. Accordingly, the present invention also discloses the manufacturing method of the above-mentioned weakly deoxidized Q355 grade seawater corrosion resistant steel, which includes the following steps: (1) smelting: hot iron 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 1000℃, the rough rolling finish temperature to be not less than 950℃, the intermediate billet thickness to be 3 to 5 times the finished product thickness, the finish rolling start temperature to be not greater than 900℃, and the finish rolling finish temperature to be 700℃ to 800℃; (5) cooling: controlling the start cooling temperature to be ≥680℃, the cooling rate to be ≤10℃ / 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 we all know, the development of marine resources is crucial to a nation's energy security and national defense. The development of marine resources often requires the construction of large-scale facilities, such as large offshore platforms.

[0003] To ensure the corrosion resistance and service life of large offshore platforms, those skilled in the art typically use steel with seawater corrosion resistance for manufacturing. In addition to good low-temperature toughness and a low yield strength ratio, such steel plates used in marine environments also require excellent seawater corrosion resistance.

[0004] Currently, the manufacturing cost of existing seawater corrosion resistant steel plates is very high. Most of them contain a large amount of corrosion-resistant alloying elements. This alloy design greatly increases the overall engineering manufacturing and maintenance costs and affects its widespread use. It cannot meet the requirements of the industry's rapid development.

[0005] For example, Chinese patent document CN110157982A, published on August 23, 2019, entitled "A Seawater Corrosion-Resistant Steel Plate and Its Production Method," discloses a seawater corrosion-resistant steel plate and its production method. Its chemical composition, by mass percentage, is: C 0.08%–0.12%, Si 0.25%–0.35%, Mn 1.0%–1.6%, P≤0.012%, S≤0.008%, Cr+Ni+Cu≤1.2%, Sn: 0.025%–0.050%, Nb+V+Ti≤0.05%, with the remainder being Fe and unavoidable impurities. This technical solution employs low-carbon microalloying with Nb and Ti, combined with the addition of corrosion-resistant elements and the control of rolling parameters, to obtain high strength, excellent seawater corrosion resistance, and weldability. However, the addition of certain amounts of Cr, Ni, and Cu to its chemical composition, while achieving some resistance to seawater corrosion and low-temperature toughness, increases the cost of the alloy.

[0006] For example, Chinese patent document CN106498288A, published on March 15, 2017, entitled "A Ti-containing seawater corrosion-resistant steel strip and its preparation method," discloses a Ti-containing seawater corrosion-resistant steel strip and its preparation method. Its chemical composition and mass fraction are as follows: C: 0.02~0.15, Si: 0.2~0.4, Mn: 0.8~1.5, P: 0.07~0.1, S≤0.005, Cu: 0.2~0.5, Ti: 0.15~0.4, V: 0~0.06, Ni: 0~0.4, with the balance being Fe and other unavoidable impurity elements. In this technical solution, the invented steel strip has a unique composition and excellent corrosion resistance and corrosion fatigue resistance. The manufacturing method does not require tempering, allowing for hot-rolled supply. The steel plate has a yield strength above 435 MPa, a tensile strength above 540 MPa, an elongation greater than 35%, and a Charpy impact energy greater than 48 J at -40℃. However, this technical solution involves the addition of certain amounts of Cu, Ni, Ti, and V to the chemical composition. While this achieves better seawater corrosion resistance and low-temperature toughness, it increases the alloy cost.

[0007] For example, Chinese patent document CN106756593A, published on May 31, 2017, entitled "A Seawater Corrosion-Resistant Steel and Its Manufacturing Method," discloses a seawater corrosion-resistant steel and its manufacturing method. Its chemical composition and weight percentages are: C: 0.04–0.07, Si: 0.30–0.40, Mn: 0.60–0.90, P≤0.015, S≤0.005, Cu: 0.20–0.30, Cr: 0.80–1.00, Mo: 0.15–0.25, Nb: 0.01–0.02, Ti: 0.01–0.03, Al: 0.02–0.06, Ca: 0.002–0.006, with the remainder being Fe and other unavoidable impurity elements. This technical solution employs a low-C, low-Mn chemical composition design and adds a certain amount of Cu, Cr, Mo, Nb, and Ti. While it can achieve certain resistance to seawater corrosion and low-temperature toughness, it leads to an increase in alloy cost. Summary of the Invention

[0008] One objective of this invention is to provide a weakly deoxidized Q355 grade seawater corrosion-resistant steel. This steel plate employs a rationally designed chemical composition, achieving excellent seawater corrosion resistance while controlling manufacturing costs. The steel plate exhibits a yield strength ≥345MPa at room temperature, a tensile strength of 470~630MPa, an elongation ≥22%, an impact energy (KV2) ≥120J at -20℃, and a hardness ≤200HBW. Its annual corrosion rate in saturated NaCl aqueous solution is ≤1.3mm / year. It can be used to manufacture structural components for offshore platforms and marine environments, possessing significant practical value.

[0009] To achieve the above objectives, the present invention provides a weakly deoxidized Q355 grade seawater corrosion resistant steel, which contains Fe and unavoidable impurity elements, and also contains the following chemical elements in the following mass percentages:

[0010] C: 0.05~0.15%, 0<Si≤0.15%, Mn: 0.80~1.50%, P: 0.030~0.080%, Cr: 0.20~0.80%, Nb: 0 .010~0.050%, Ti: 0.010~0.020%, O: 0.010~0.025%, Ce: 0.020~0.040%, 0<Als≤0.010%.

[0011] Furthermore, in the weakly deoxidized Q355 grade seawater corrosion-resistant steel described in this invention, the mass percentage content of each chemical element is as follows:

[0012] C: 0.05–0.15%, 0 < Si ≤ 0.15%, Mn: 0.80–1.50%, P: 0.030–0.080%, Cr: 0.20–0.80%, Nb: 0.010–0.050%, Ti: 0.010–0.020%, O: 0.010–0.025%, Ce: 0.020–0.040%, 0 < Als ≤ 0.010%; the balance is Fe and unavoidable impurity elements.

[0013] In the design process, the inventors considered that the steel of this invention mainly aims to obtain excellent strength, toughness and seawater corrosion resistance. Therefore, in the actual smelting and steelmaking process, it is necessary to accurately control the oxygen content of the molten steel and remove large-sized inclusions caused by high oxygen content, thereby reducing defects such as surface porosity of the billet during continuous casting.

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

[0015] The design principles of each chemical element in the weakly deoxidized Q355 grade seawater corrosion resistant steel described in this invention are as follows:

[0016] C: In the weakly deoxidized Q355 grade seawater 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 0.05% and 0.15%.

[0017] Si: In the weakly deoxidized Q355 grade seawater corrosion resistant steel described in this invention, Si has a very weak affinity for C and does not combine with C in the steel. However, Si can dissolve into ferrite and produce a solid solution strengthening effect, which increases the strength and hardness of ferrite, but at the same time, it also reduces the plasticity and toughness of the steel. Although Si contributes to the strength of the material, it also promotes the formation of island martensite, which is detrimental to the toughness of the weld heat-affected zone. Therefore, the amount of Si added to the steel should not be too high. In addition, as a deoxidizer, Si has a stronger deoxidizing ability than Mn. When the Si content in the steel increases, it will also lead to a decrease in the O content in the steel. Therefore, in the weakly deoxidized Q355 grade seawater corrosion resistant steel described in this invention, the mass percentage of Si is controlled to be 0 < Si ≤ 0.15%.

[0018] Mn: In the weakly deoxidized Q355 grade seawater 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 comprehensive performance, 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 also 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 1.50% when designing this steel. Thus, considering the strength range of the steel, the mass percentage of Mn in the weakly deoxidized Q355 grade seawater corrosion resistant steel described in this invention is controlled between 0.80% and 1.50%.

[0019] P: In the weakly deoxidized Q355 grade seawater 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. Able to interact with 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 greatest drawback is severe segregation, increasing temper brittleness and significantly reducing the plasticity and toughness of the steel. Therefore, considering all factors, in the weakly deoxidized Q355 grade seawater corrosion-resistant steel described in this invention, the mass percentage of phosphorus is strictly controlled between 0.030% and 0.080%.

[0020] Cr: In the weakly deoxidized Q355 grade seawater corrosion-resistant steel described in this invention, Cr is a commonly used element in corrosion-resistant steel. Compared to ordinary carbon steel and low-alloy steel, the enrichment of Cr in the formed corrosion film can improve the stability of the corrosion film and effectively delay corrosion deterioration. Therefore, considering alloy cost and usage requirements, in this invention, the mass percentage of Cr is controlled between 0.20% and 0.80%.

[0021] Nb: In the weakly deoxidized Q355 grade seawater corrosion resistant steel described in this invention, Nb is an element that significantly improves the dynamic recrystallization of austenite. It can effectively refine the matrix grains and, in conjunction with a large reduction during rolling, significantly refine the austenite grains. Adding an appropriate amount of Nb to the steel can effectively refine the grains and simultaneously improve the strength and low-temperature toughness of the steel plate. However, it should be noted that Nb should not be added in excess. Too much Nb in the steel will cause the size of the second phase particles to increase and will affect the weldability of the steel. Therefore, in the weakly deoxidized Q355 grade seawater corrosion resistant steel described in this invention, the mass percentage of Nb is controlled between 0.010% and 0.050%.

[0022] Ti: In the weakly deoxidized Q355 grade seawater corrosion resistant steel described in this invention, Ti is a strong carbide and nitride forming element. Ti particles such as TiN and Ti(CN) formed in the steel are very stable and can effectively inhibit grain growth during nucleation, thus refining the grains and improving the strength and toughness of the steel plate. However, Ti's contribution to strength is less significant than that of Nb, and excessive Ti carbides can also reduce the low-temperature toughness of the steel plate. Therefore, considering the strength and toughness requirements of the steel plate and its combination with other elements, the mass percentage of Ti is controlled between 0.010% and 0.020% in this invention.

[0023] O: In the weakly deoxidized Q355 grade seawater corrosion resistant steel described in this invention, the content of oxygen (O) in the steel must be strictly controlled. O is generally considered a harmful gas element in steel. Excessive O content easily generates more oxygen-containing inclusions, causing defects such as surface 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 and make them ellipsoidal or granular, thereby reducing stress concentration at the inclusion tips and 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 also beneficial for enhancing the corrosion resistance of the steel plate. Based on this, considering the comprehensive influence of O content on the steel's properties, the mass percentage of O in the weakly deoxidized Q355 grade seawater corrosion resistant steel described in this invention is strictly controlled between 0.010% and 0.025%.

[0024] Ce: In the weakly deoxidized Q355 grade seawater 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 is too high, rare earth inclusions are easily formed and aggregated, 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%.

[0025] Al: In the weakly deoxidized Q355 grade seawater 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; an increase in the Al content will significantly reduce the O content. Therefore, in the weakly deoxidized Q355 grade seawater corrosion resistant steel described in this invention, the Al content must be strictly controlled, maintaining it at 0 < Als ≤ 0.010%.

[0026] Furthermore, in the weakly deoxidized Q355 grade seawater corrosion resistant steel described in this invention, among the unavoidable impurity elements, S ≤ 0.002%.

[0027] In the above technical solution, S is an impurity element in the weakly deoxidized Q355 grade seawater 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.

[0028] 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 weakly deoxidized Q355 grade seawater corrosion-resistant steel described in this invention, the S content in the steel needs to be strictly controlled, and the mass percentage of S must be controlled to meet the requirement: S ≤ 0.002%.

[0029] Furthermore, in the weakly deoxidized Q355 grade seawater corrosion resistant steel described in this invention, each chemical element satisfies at least one of the following formulas:

[0030] 3≤Si / O≤10;

[0031] 8≤Mn / Si≤20;

[0032] CEV≤0.43, where CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15, and each chemical element is represented by the value before the percentage sign of its mass percentage content.

[0033] 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 3 ≤ Si / O ≤ 10, wherein the values ​​of the above 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.

[0034] Accordingly, the weakly deoxidized Q355 grade seawater corrosion-resistant steel of the present invention, while controlling the mass percentage content of a single chemical element in the steel, can also control the Mn and Si elements to satisfy the limiting relationship of 8 ≤ Mn / Si ≤ 20, where each chemical element is substituted with the value before the percentage sign of its mass percentage content. This limiting relationship can ensure the range of Si and Mn content during the steel smelting process, guaranteeing the weak deoxidation effect and the contribution of Si and Mn to the strength of the steel plate.

[0035] Furthermore, in the design of chemical composition, the present invention can further control CEV≤0.43 to ensure the weldability of steel plates and reduce the tendency of welding cracks.

[0036] Furthermore, in the weakly deoxidized Q355 grade seawater corrosion resistant steel described in this invention, its microstructure is ferrite + pearlite, wherein the proportion of ferrite is more than 60%.

[0037] Furthermore, in the weakly deoxidized Q355 grade seawater corrosion resistant steel of the present invention, it has composite inclusions, the center of which is Al2O3, TiO, MnO and SiO2, and the surface is Ce2O2S, Ce2S3 or CeS.

[0038] Furthermore, in the weakly deoxidized Q355 grade seawater corrosion resistant steel described in this invention, its properties meet the following requirements: yield strength at room temperature ≥345MPa, tensile strength 470~630MPa, elongation ≥22%; impact energy KV2 at -20℃ ≥120J; hardness ≤200HBW; and annual corrosion rate measured in saturated NaCl aqueous solution ≤1.3mm / year.

[0039] Furthermore, in the weakly deoxidized Q355 grade seawater corrosion resistant steel described in this invention, its yield strength at room temperature is ≥355MPa.

[0040] Accordingly, another objective of the present invention is to provide a method for manufacturing the weakly deoxidized Q355 grade seawater corrosion resistant steel described above. This manufacturing method has an optimized process design and a low manufacturing cost. The weakly deoxidized Q355 grade seawater corrosion resistant steel obtained by this manufacturing method has excellent mechanical properties as well as good resistance to marine atmospheric corrosion.

[0041] To achieve the above objectives, the present invention proposes a method for manufacturing the aforementioned weakly deoxidized Q355 grade seawater corrosion-resistant steel, comprising the following steps:

[0042] (1) Smelting: hot metal pretreatment, converter smelting, argon protection and LF furnace treatment;

[0043] (2) Continuous casting, after pouring, the billets are stacked and cooled slowly;

[0044] (3) Heating;

[0045] (4) Rolling: The roughing rolling start temperature is not less than 1000℃, the roughing rolling finish temperature is not less than 950℃, the intermediate billet thickness is 3 to 5 times the finished product thickness, the finishing rolling start temperature is not greater than 900℃, and the finishing rolling finish temperature is 700℃ to 800℃.

[0046] (5) Cooling: Control the cooling temperature to be ≥680℃, the cooling rate to be ≤10℃ / s, and the red temperature to be ≤600℃.

[0047] Based on an optimized chemical composition, this invention further optimizes and designs a reasonable manufacturing method. In this method, a controlled rolling and cooling (TMCP) process is used for the heated billet, and the process parameters of the TMCP process are strictly controlled to obtain a stable ferrite + pearlite microstructure, thereby ensuring that the steel obtains good strength, toughness, and resistance to marine atmospheric corrosion.

[0048] The controlled cooling process (TMCP) of this invention specifically includes the rolling and cooling processes described in steps (4) and (5) above. Specifically, 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 time, the rolling of the non-recrystallized austenite region has a sufficient compression ratio, resulting in high-distortion deformation accumulation in the deformed austenite, forming a large number of 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.

[0049] This rolling process ensures that the steel plate's strength and toughness meet the design requirements. Therefore, considering all factors, in the rolling process of step (4) of this invention, the roughing rolling start temperature is controlled to be no less than 1000℃, the roughing rolling finish temperature is no less than 950℃, the finishing rolling start temperature is no greater than 900℃, and the finishing rolling finish temperature is 700℃~800℃. At the same time, in order to ensure that the grains are sufficiently reduced, the thickness of the intermediate billet is set to 3 to 5 times the thickness of the finished product.

[0050] Accordingly, according to the strength grade requirements of the steel according to the present invention, under the rolling deformation conditions set in the process, the hardness of the steel increases with the increase of the cooling rate. Therefore, in the present invention, with the statistical average hardness of the steel plate designed as 200 HBW as the target hardness value, the cooling rate to obtain the ideal hardness level of the steel of the present invention should be less than or equal to 10℃ / s. Under this condition, the microstructure of the steel plate is ferrite + pearlite, and the proportion of ferrite is more than 60%. Therefore, in the cooling step of step (5) above, the cooling rate needs to be strictly controlled to ≤10℃ / s.

[0051] In addition, it should be noted that in the smelting process of step (1) of the present invention, hot metal desulfurization technology can be used for hot metal pretreatment, and then a converter can be used for smelting, top and bottom blowing of the converter, and Si-Mn deoxidation treatment in the converter, so as to modify the oxygen-containing inclusions, effectively change the shape of the inclusions, and thus improve the pitting corrosion resistance of the steel.

[0052] 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 oxygen from escaping and causing the oxygen content in the steel to fail to reach the target value.

[0053] Accordingly, in step (2) above, the molten steel after smelting needs to be continuously cast to obtain a billet. During the continuous casting process, rare earth wire is added to modify the inclusions, and the billet is stacked and cooled slowly after casting.

[0054] Furthermore, in the manufacturing method described in this invention, in step (3), the heating temperature is controlled at 1100-1200℃ and the heating rate is controlled at 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 1100 to 1200°C and the heating rate is controlled at 8 to 15 min / cm, so as to ensure that the billet temperature is uniform, reduce the iron oxide scale of the billet, and improve the surface quality 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 1000℃, the roughing rolling finish temperature is controlled to be no less than 950℃, the intermediate billet thickness is 40 to 60 mm, the finishing rolling start temperature is no greater than 900℃, the finishing rolling passes are 6 to 8, and the finishing rolling finish temperature is 750℃ to 800℃; in step (5), the cooling start temperature is controlled to be ≥700℃, the cooling rate is controlled to be ≤10℃ / s, and the reddening temperature is controlled to be ≤600℃.

[0058] Furthermore, in the manufacturing method described in this invention, 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 1000℃, the roughing rolling finish temperature is controlled to be no less than 950℃, the intermediate billet thickness is 60 to 90mm, the finishing rolling start temperature is no greater than 880℃, the finishing rolling passes are 5 to 6, and the finishing rolling finish temperature is 700℃ to 750℃; in step (5), the cooling start temperature is controlled to be ≥680℃, the cooling rate is ≤7℃ / s, and the reddening temperature is ≤580℃.

[0059] The weakly deoxidized Q355 grade seawater 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 seawater corrosion-resistant steels 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 weakly deoxidized Q355 grade seawater corrosion-resistant steel. This steel has lower manufacturing costs, is suitable for large-scale production operations, meets market demands, and has significant economic and environmental benefits.

[0061] In designing the chemical element composition, this invention employs a low-C, low-Si, high-P, and high-O design, while controlling the addition of certain amounts of Mn, Cr, Nb, Ti, and Ce elements, and strictly controlling the content of the impurity element S, to ensure that the steel achieves excellent seawater corrosion resistance. 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 finally prepared by this invention has good strength and toughness, as well as excellent resistance to seawater corrosion. Its yield strength at room temperature is ≥345MPa, tensile strength is 470~630MPa, elongation is ≥22%; impact energy at -20℃ KV2≥120J; hardness ≤200HBW; and annual corrosion rate measured in saturated NaCl aqueous solution is ≤1.3mm / year.

[0063] Therefore, this invention, through composition design, inclusion control, rolling, and cooling, can obtain steel with excellent resistance to marine atmospheric corrosion while controlling manufacturing costs. The production process of this weakly deoxidized Q355 grade seawater corrosion-resistant steel is simple and feasible, suitable for large-scale production. It can be used to manufacture various large marine platform structural components and marine climate 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 weakly deoxidized Q355 grade seawater corrosion resistant steel of Example 3 under a 200x optical electron microscope. Detailed Implementation

[0065] The following will further explain and illustrate the weakly deoxidized Q355 grade seawater 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 improper limitation on the technical solution of the present invention.

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

[0067] The weakly deoxidized Q355 grade seawater 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: Rare earth wire is added during continuous casting, and the billet is stacked and cooled slowly after pouring.

[0070] (3) Heating: Control the heating temperature to 1100~1200℃ and the heating rate to 8~15min / cm.

[0071] (4) Rolling: Control the roughing rolling start temperature to be no less than 1000℃, control the roughing rolling finish temperature to be no less than 950℃, control the intermediate billet thickness to be 3 to 5 times the finished product thickness, control the finishing rolling start temperature to be no greater than 900℃, control the finishing rolling finish temperature to be 700℃ to 800℃, and control the roughing rolling pass reduction rate to be ≥12%, the finishing rolling pass reduction rate to be ≥10%, the finishing rolling last three passes reduction rate to be ≥30%, and the finishing rolling total reduction rate to 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 1000℃, the roughing rolling finish temperature to be no less than 950℃, the intermediate billet thickness to be 40 to 60 mm, the finishing rolling start temperature to be no greater than 900℃, the finishing rolling passes to be 6 to 8, and the finishing rolling finish temperature to be 750℃ to 800℃.

[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 1000℃, the roughing rolling finish temperature to be no less than 950℃, the intermediate billet thickness to be 60-90mm, the finishing rolling start temperature to be no greater than 880℃, the finishing rolling passes to be 5-6, and the finishing rolling finish temperature to be 700℃-750℃.

[0074] (5) Cooling: Control the cooling temperature to be ≥680℃, the cooling rate to be ≤10℃ / s, and the red temperature to be ≤600℃. For steel plates with a finished thickness of 6–16 mm, the initial cooling temperature can be preferably controlled at ≥700℃, and specifically the water flow rate in the weak cooling zone can be controlled at 50 L / s, the water flow rate in the weak cooling zone at 100 L / s, the water flow rate in the strong cooling zone at 100 L / s, the water flow rate in the strong cooling zone at 400 L / s, the roller speed at 0.7 m / s, the cooling rate at ≤10℃ / s, and the reddening temperature at ≤600℃. For steel plates with a finished thickness of 16 mm to 30 mm, the initial cooling temperature can be preferably controlled at ≥680℃, and specifically the water flow rate in the weak cooling zone can be controlled at 60 L / s, the water flow rate in the weak cooling zone at 100 L / s, the water flow rate in the strong cooling zone at 120 L / s, the water flow rate in the strong cooling zone at 400 L / s, the roller speed at 0.7 m / s, the cooling rate at ≤7℃ / s, and the reddening temperature at ≤580℃.

[0075] (6) Normalizing: The normalizing temperature is controlled at 870~940℃, and the holding time is controlled at (20~40min)+t×1min / mm, where t represents the plate thickness and its unit parameter is mm.

[0076] It should be noted that, in actual experiments, after completing the above steps (1)-(5), the weakly deoxidized Q355 grade seawater corrosion resistant steel 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 weakly deoxidized Q355 grade seawater corrosion resistant steel in Examples 1-6 all meet the design specifications of this invention.

[0077] 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 process. Moreover, Comparative Examples 1-2 have parameters in chemical composition design and related processes that do not meet the design requirements of the present invention.

[0078] Tables 1-1 and 1-2 list the mass percentage of each chemical element in the weakly deoxidized Q355 grade seawater corrosion resistant steel of Examples 1-6 and the comparative steel plates of Comparative Examples 1-2.

[0079] Table 1-1. (wt%, balance Fe and other unavoidable impurities besides S)

[0080]

[0081] Table 1-2.

[0082] serial number Si / O Mn / Si CEV Example 1 10 8 0.37 Example 2 9 12 0.38 Example 3 5 11 0.43 Example 4 4 15 0.42 Example 5 3 19 0.41 Example 6 3 18 0.42 Comparative Example 1 — — — Comparative Example 2 — — —

[0083] Note: In the table above, CEV = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, 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 weakly deoxidized Q355 grade seawater 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 weakly deoxidized Q355 grade seawater 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 weakly deoxidized Q355 grade seawater corrosion resistant steel of Examples 1-6 was ferrite + pearlite, and the proportion of ferrite was more than 60%.

[0090] Furthermore, composite inclusions were found in the weakly deoxidized Q355 grade seawater 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 test results of the composite inclusions in Table 3 below.

[0091] Table 3 lists the composite inclusion detection results of the weakly deoxidized Q355 grade seawater 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, the composite inclusions in the weakly deoxidized Q355 grade seawater corrosion resistant steel of Examples 1-6 and the comparative steel plates of Comparative Examples 1-2 can be observed. Compared with Comparative Examples 1-2, the inclusions in Examples 1-6 are of lower grade and smaller size, which improves the corrosion resistance of the steel plates.

[0095] Observation of the inclusions in the weakly deoxidized Q355 grade seawater corrosion resistant steel of Examples 1-6 shows that the center of the composite inclusions in the weakly deoxidized Q355 grade seawater 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 steel in each example and comparative example at room temperature.

[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 each embodiment and comparative example steel 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 under the cyclic immersion accelerated corrosion test, so as to obtain the annual corrosion rate.

[0102] Table 4 lists the observation results and mechanical property test results of the weakly deoxidized Q355 grade seawater corrosion resistant steel of Examples 1-6 and the comparative steel plates of Comparative Examples 1-2.

[0103] Table 4.

[0104]

[0105] As can be seen from Table 4, compared with the comparative steel plates of Comparative Examples 1-2, the weakly deoxidized Q355 grade seawater corrosion resistant steel of Examples 1-6 has significantly better overall performance, especially better corrosion resistance.

[0106] The weakly deoxidized Q355 grade seawater corrosion resistant steel described in this invention has a yield strength between 410-450 MPa, a tensile strength between 520-560 MPa, an elongation between 27-30% at room temperature, an impact energy (KV2) between 120-210 J at -20℃, a hardness between 170-190 HBW, and an annual corrosion rate of 1.10-1.20 mm / year measured in saturated NaCl aqueous solution.

[0107] Therefore, the steel of this invention has excellent mechanical properties and excellent resistance to seawater corrosion. It can be used to manufacture various large marine platform structural components and marine atmospheric environment building structural components, and meets market demand, which has great practical significance.

[0108] Figure 1 The image shows the metallographic structure of the weakly deoxidized Q355 grade seawater 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 weakly deoxidized Q355 grade seawater corrosion-resistant steel of Example 3 is ferrite + pearlite, with the ferrite phase ratio being above 60%. Correspondingly, in this embodiment, 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 weakly deoxidized Q355 grade seawater corrosion resistant steel, characterized in that, Its mass percentage content of each chemical element is as follows: C: 0.05~0.15%, 0<Si≤0.15%, Mn: 0.80~1.50%, P: 0.030~0.080%, Cr: 0.20~0.80%, Nb: 0.010~0.050%, Ti: 0.010~0.020%, O: 0.010~0.025%, Ce: 0.020~0.040%, 0<Als≤0.010%, balance being Fe and unavoidable impurity elements; The weakly deoxidized Q355 grade seawater corrosion resistant steel has composite inclusions, the center of which is Al2O3, TiO, MnO and SiO2, and the surface is Ce2O2S, Ce2S3 or CeS.

2. The weakly deoxidized Q355 grade seawater corrosion resistant steel as described in claim 1, characterized in that, In unavoidable impurity elements, S ≤ 0.002%.

3. The weakly deoxidized Q355 grade seawater corrosion resistant steel as described in claim 1, characterized in that, Each chemical element satisfies at least one of the following formulas: 3≤Si / O≤10; 8≤Mn / Si≤20; CEV≤0.43, where CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15, and each chemical element is represented by the value before the percentage sign of its mass percentage content.

4. The weakly deoxidized Q355 grade seawater corrosion resistant steel as described in claim 1, characterized in that, Its microstructure consists of ferrite and pearlite, with ferrite accounting for more than 60%.

5. The weakly deoxidized Q355 grade seawater corrosion resistant steel as described in claim 1, characterized in that, Its performance meets the following requirements: yield strength at room temperature ≥345MPa, tensile strength 470~630MPa, elongation ≥22%; impact energy KV2 ≥120J at -20℃; hardness ≤200HBW; and annual corrosion rate ≤1.3mm / year measured in saturated NaCl aqueous solution.

6. The weakly deoxidized Q355 grade seawater corrosion resistant steel as described in claim 5, characterized in that, Its yield strength at room temperature is ≥355MPa.

7. The method for manufacturing weakly deoxidized Q355 grade seawater 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 1000℃, the roughing rolling finish temperature is not less than 950℃, the intermediate billet thickness is 3 to 5 times the finished product thickness, the finishing rolling start temperature is not greater than 900℃, and the finishing rolling finish temperature is 700℃ to 800℃. (5) Cooling: Control the starting temperature of cooling to ≥680℃, the cooling rate to ≤10℃ / s, and the temperature at which the temperature turns red to ≤600℃.

8. The manufacturing method as described in claim 7, characterized in that, In step (3), the heating temperature is controlled at 1100~1200℃ 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 1000℃, the roughing rolling finish temperature is controlled to be no less than 950℃, the intermediate billet thickness is 40 to 60 mm, the finishing rolling start temperature is no greater than 900℃, the finishing rolling passes are 6 to 8 times, and the finishing rolling finish temperature is 750℃ to 800℃; in step (5), the cooling start temperature is controlled to be ≥700℃, the cooling rate is ≤10℃ / 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 1000℃, the roughing rolling finish temperature is controlled to be no less than 950℃, the intermediate billet thickness is 60~90mm, the finishing rolling start temperature is no greater than 880℃, the finishing rolling passes are 5~6 times, and the finishing rolling finish temperature is 700℃~750℃; in step (5), the cooling start temperature is controlled to be ≥680℃, the cooling rate is ≤7℃ / s, and the reddening temperature is ≤580℃.

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