Corrosion-resistant steel plate for bottom plate of cargo oil tank of crude oil tanker and manufacturing method thereof

Abstract

A corrosion-resistant steel plate for a bottom plate of a cargo oil tank of a crude oil tanker and a manufacturing method thereof. The corrosion-resistant steel plate comprises components in percentage by weight: C: 0.010-0.10%, Si: 0.10-0.50%, Mn: 0.5-1.5%, P: 0.008-0.060%, S≤0.010%, Cr: 0.5-2.0%, Mo: 0.5-1.50%, Cu: 0.5-2.00%, Ni: 0.5-2.50%, Al: 0.01-0.12%, Ti: 0.005-0.15%, and the balance comprising Fe and inevitable impurities, and the elements also need to meet: Cr≤(39+300Cu+590Ni) / 100. The steel plate has a yield strength greater than or equal to 355 MPa, a tensile strength of 500-700 MPa, an elongation rate greater than or equal to 35%, an impact energy at -60°C greater than or equal to 300 J, and an average corrosion rate CR smaller than or equal to 0.8 mm / year. Obtained is a corrosion-resistant steel used for a bottom plate in a cargo oil tank of a crude oil tanker and having excellent uniform corrosion resistance and pitting corrosion resistance, and the adverse impact of a traditional mode of adding W to improve corrosion resistance on the welding performance of steel plates is avoided.

Description

Corrosion-resistant steel plate for crude oil tanker cargo tank bottom plate and manufacturing method thereof TECHNICAL FIELD

[0001] The present application relates to the technical field of ship steel plate, in particular to a corrosion-resistant steel plate for crude oil tanker cargo tank bottom plate and a manufacturing method thereof. BACKGROUND

[0002] As a strategic material, the emergence of energy crisis further highlights the importance of the development, transportation and utilization of crude oil resources worldwide. Marine transportation is the main form of crude oil transportation and one of the important production processes. In recent years, due to a number of marine oil spill accidents, it has caused great influence on the human living environment and marine ecological balance, so the safety performance of crude oil tanker transportation has attracted much attention. In view of the safety, energy saving and environmental protection requirements of crude oil tanker operation, the International Maritime Organization (IMO) has increasingly strict requirements for the corrosion protection of crude oil tanker cargo tank, and has put forward the standard for corrosion protection of crude oil tanker cargo tank. At the same time, it puts forward the application of corrosion-resistant steel as an effective alternative measure to improve the corrosion resistance of cargo tank. As a corrosion protection measure to replace coating protection for crude oil tanker cargo tank, corrosion-resistant steel has the obvious advantages of economy, environmental protection and energy saving, and is the most important development trend of crude oil tanker steel plate.

[0003] For large ships transporting crude oil, the corrosion of oil tanker plate greatly shortens the service life of oil tanker and directly threatens the safety of the ship, so the corrosion resistance of the plate is a decisive factor for the service life of the ship. According to the investigation of the corrosion of crude oil in the cargo tank of crude oil tanker, it is found that in the actual cargo tank of large oil tanker, the corrosion is mainly caused by hydrogen sulfide and high concentration salt solution separated from the oil, forming a point pit corrosion. With the development of double hull, the pitting corrosion of oil tanker bottom plate becomes a significant problem. This is because under the single hull, the skeleton of the ship body is protruding, while under the double hull, the skeleton of the ship body is hidden inside the double hull, and the bottom plate is in a flat structure. Originally, the thick oil film formed by the solidification of the oil in the tank plays a role in corrosion protection, but the oil injection used for cleaning makes the oil film on the double hull bottom plate thin, which is easy to cause pitting corrosion. In addition, due to the fact that the salt concentration in the crude oil is many times higher than that in seawater, over time, salt water accumulates on the inner bottom plate of the oil tank, which will cause the corrosion of the inner surface of the oil tank.

[0004] The bottom of the crude oil tank (COT) forms a thick oil covering layer after the first loading of crude oil. The steel material covered by oil is not prone to corrosion. However, the oil covering layer is easily damaged due to the regular use of high-pressure crude oil cleaning inside the COT. In addition, in the crude oil inside the COT, high-concentration rock salt and dissolved underground water mixed together exist in the oil well. The rock salt precipitates and remains in the bottom during the crude oil transportation process, becoming the main cause of the bottom plate corrosion. The diameter of the pitting corrosion on the inner bottom plate is mostly 10-30 mm, the depth is mostly 1-6 mm, the maximum depth is up to 10 mm, and the propagation rate is 2-3 mm / year. With the formation of the micro-battery reaction of the pitting corrosion pit, the acidity in the pitting corrosion pit becomes larger and larger, and the pH value gradually decreases to be lower than the pH value outside the pit. The minimum pH value can be as low as 0.85. With the decrease of the pH value, the pitting corrosion rate increases significantly, and the maximum cargo oil tank pitting corrosion rate can be up to 3-4 mm / year.

[0005] The most effective protection method for the cargo oil tank bottom plate corrosion as described above is coating protection. However, due to the large coating area of the cargo oil tank, and the need for re-coating every 10 years to maintain the protective effect of the coating, a large amount of construction and detection maintenance costs will be caused. In addition, even if coating is implemented on the bottom plate, regular tank cleaning maintenance will cause coating defects, which are the main cause of the formation of pitting corrosion pits. If the corrosion thickness allowance is increased to reduce corrosion, it will bring adverse factors to the safety and maintenance cost of the oil tanker operation, and will also reduce the oil loading capacity of the ship and increase the construction cost of the oil tanker. Therefore, an economic, environmentally friendly and convenient corrosion protection measure is actively sought, and corrosion-resistant steel without coating is undoubtedly the best choice.

[0006] According to the corrosion environment and corrosion mechanism of the cargo oil tank of the crude oil tanker, the corrosion resistance of the corrosion-resistant steel inner bottom plate of the cargo oil tank of the crude oil tanker mainly reflects the pitting corrosion resistance in the strong acidic environment. To improve the pitting corrosion resistance of the steel plate, the composition design must enable the steel plate surface to quickly form a stable and dense acid-resistant environment corrosion product layer in the actual environment, hinder the further oxidation corrosion of the steel plate matrix, and remain stable in the extremely low pH acidic environment. In addition, the content and form of sulfides and other inclusions in the steel must be strictly controlled, and the organization state, alloy elements, grain size and microstructure, phase proportion, etc. must also be controlled to hinder the direct corrosion reaction of the steel plate organization in the high-acid environment.

[0007] Chinese patent CN101415852A discloses a method for manufacturing a corrosion-resistant steel material for crude oil tanks, the alloy composition of the steel material is: C: 0.001-0.16%, Si: 0.01-1.5%, Mn: 0.1-2.5%, P≤0.025%, S≤0.01%, Al: 0.005-0.1%, Cr: 0.06-0.20%, W: 0.001-0.5%, N: 0.001-0.008%, mainly adding W element to increase corrosion resistance, which can reduce the local corrosion occurring at the bottom plate or the overall corrosion occurring at the deck and side plate.

[0008] Chinese patent CN1662668A discloses a steel for crude oil tanks and a method for manufacturing the same, a crude oil tank and a method for preventing corrosion of the same, the alloy composition of the steel material is: C: 0.001-0.2%, Si: 0.01-2.5%, Mn: 0.1-2%, P≤0.03%, S≤0.007%, Al: 0.001-0.3%, Cu: 0.01-1.5%, N: 0.001-0.01%, and further contains at least one of W: 0.001-0.5% and Mo: 0.01-0.2%, and further preferably satisfies solid solution Mo+solid solution W≥0.005%, and the alloy composition design adopts adding W element to increase corrosion resistance. The steel for crude oil tanks shows excellent overall corrosion resistance and local corrosion resistance, and can inhibit the generation of corrosion products (sludge) containing solid S.

[0009] US patent US20100003161A1 discloses a corrosion-resistant steel plate for crude oil tankers, which adopts C-Mn steel with Cu, Mo, W alloying treatment, Al, N control, and preferably satisfies solid solution Mo+solid solution W≥0.005%, and also needs to add W to improve corrosion resistance. It can inhibit the generation of corrosion products containing solid sulfides, prevent the oil tank from being corroded and locally corroded. Moreover, the patent is aimed at ordinary seawater and oil sewage corrosion, and does not specially target the corrosion environment of the bottom plate of the crude oil tanker cargo oil tank.

[0010] At present, the performance of seawater corrosion-resistant steel plates is improved by adding W to improve corrosion resistance, but the addition of W will have a negative impact on the welding performance of the steel plate, therefore, there are limitations in production and welding matching applications. SUMMARY

[0011] The present application aims to provide a kind of crude oil ship cargo oil tank bottom plate corrosion-resistant steel plate and its manufacturing method, the yield strength of the steel plate is ≥355MPa, tensile strength is 500~700MPa, elongation is ≥35%, impact energy at-60 ℃ is ≥300J, under the simulation of crude oil ship cargo oil tank inner bottom plate PH value equal to 0.85 severe acidic environment, average corrosion rate CR≤0.8mm / year, obtain the steel plate with excellent uniform corrosion resistance and local pitting corrosion resistance, can be used to manufacture large crude oil ship cargo oil tank bottom plate structure and other marine, ship corrosion-resistant structure etc.

[0012] To achieve the above purpose, in the first aspect, the present application provides a kind of crude oil ship cargo oil tank bottom plate steel plate, in addition to Fe and inevitable impurities, the steel plate also contains the following weight percentage of ingredients: C:0.010~0.10%, Si:0.10~0.50%, Mn:0.5~1.5%, P:0.008~0.060%, S≤0.010%, Cr:0.5~2.0%, Mo:0.5~1.50%, Cu:0.5~2.00%, Ni:0.5~2.50%, Al:0.01~0.12%, Ti:0.005~0.15%, the steel plate also satisfies: Cr≥(39+300Cu+590Ni) / 100, in the formula, each element symbol represents the weight percentage of corresponding element.

[0013] Preferably, the steel plate composition also includes at least one of the following: V≤0.15%, preferably 0.04%≤V≤0.15%, Nb≤0.15%, preferably 0.012%≤Nb≤0.15%, Sb≤0.15%, preferably 0.012%≤Sb≤0.15%, N≤0.010%, Zr≤0.25%, preferably 0.01%≤Zr≤0.25%, REM≤0.020%, preferably 0.002%≤REM≤0.02%.

[0014] Preferably, the steel plate has the following weight percentage of ingredients: C:0.010~0.10%, Si:0.10~0.50%, Mn:0.5~1.5%, P:0.008~0.060%, S≤0.010%, Cr:0.5~2.0%, Mo:0.5~1.50%, Cu:0.5~2.00%, Ni:0.5~2.50%, Al:0.01~0.12%, Ti:0.005~0.15%, the balance is Fe and inevitable impurities.

[0015] Preferably, the microstructure of the steel plate is ferrite + pearlite structure or ferrite + acicular ferrite structure, wherein the area ratio of ferrite structure is ≥90%, and the average grain size of ferrite is 5 μm~15 μm.

[0016] Preferably, the steel sheet has a yield strength of ≥ 355 MPa, preferably ≥ 420 MPa, a tensile strength of 500-700 MPa, an elongation of ≥ 35%, an impact energy at -60°C of ≥ 300 J, and an average corrosion rate CR of ≤ 0.8 mm / year.

[0017] In the component design of the present application:

[0018] C: C is an essential element for ensuring the strength of the steel. For the TMCP process of controlled rolling and controlled cooling, in order to stably maintain a certain strength, the lower limit of the C content is 0.010%, but as the C content increases, the cementite content will increase, which promotes the corrosion of the steel sheet in an acidic environment, and the increase of the cementite will deteriorate the welding performance, so the upper limit of the C content is limited to 0.10%. Therefore, the C content in the steel of the present application is controlled in the range of 0.01-0.10%.

[0019] Si: Si is an element required in the pre-deoxidation process of steelmaking, and can play a role in strengthening the base material, and the lower limit of the Si content is 0.10%. But when the Si content exceeds 0.50%, the toughness of the base material and the welded joint will be reduced. Therefore, the Si content in the steel of the present application is controlled in the range of 0.10-0.50%.

[0020] Mn: Mn can significantly improve the strength of the base material through solid solution strengthening, and is also an essential element for ensuring the strength of the steel, and is low in cost, so the lower limit of the Mn content is set to 0.5%. But too high Mn will deteriorate the welding performance of the steel sheet, and is easy to form center segregation of the slab, in addition, the increase of the Mn content will also significantly increase the formation of Mn inclusions in the steel, which will promote the corrosion in an acidic environment, therefore, the Mn content in the steel of the present application is controlled in the range of 0.5-1.50%.

[0021] P: P is usually an impurity element in steel. The grain boundary segregation of P will reduce the welding performance of the steel sheet, and especially when the P content exceeds 0.06%, the welding performance will be significantly reduced, so the upper limit of the P content is set to 0.06%. On the other hand, P element has the effect of improving the pitting corrosion resistance of the inner bottom plate of the cargo oil tank of the crude oil tanker, in order to improve the corrosion resistance of the steel sheet, the P content should be above 0.008%. Therefore, the P content in the steel of the present application is controlled in the range of 0.008-0.060%.

[0022] S: S is an impurity element in steel, if the S content in the steel exceeds 0.010%, the formation of MnS in the steel will increase, MnS is easy to become the starting point of corrosion and produce pitting corrosion, therefore, the S content should be controlled below 0.010%. The lower the S content is controlled, the better, but it will increase the production cost.

[0023] Al: Al is an element added as a deoxidizer, in the present application, Al is added above 0.01%, but if the content of Al exceeds 0.12%, the toughness of the steel plate will be reduced, so the upper limit of the content of Al is set to 0.12%. Therefore, the content of Al in the steel of the present application is controlled in the range of 0.01-0.12%.

[0024] Ni: Ni is a necessary element in the steel to ensure low-temperature toughness, and Ni also has the effect of improving the pitting corrosion resistance of the inner bottom plate of the cargo oil tank of the crude oil tanker. Ni can promote the thermal stability of the microstructure of the steel plate, delay the occurrence of pitting corrosion of the steel plate, and in an acidic environment, Ni and Cu couple to gather and thicken at the corrosion interface to form a passivation layer, thereby improving the corrosion resistance of the steel plate in an acidic environment. These effects are significantly manifested when the content of Ni is above 0.5%. However, when the content of Ni is too high, the additive effect of the content of Ni is not significant, and the production cost is increased. Therefore, the content of Ni in the steel of the present application is controlled in the range of 0.5-2.50%.

[0025] Cu: Cu is an element that improves the pitting corrosion resistance of the inner bottom plate of the cargo oil tank of the crude oil tanker. In an acidic environment, Cu can promote the formation of a stable passivation film protective layer on the surface of the steel plate, delay the occurrence of pitting corrosion of the steel plate, and Cu and Ni couple to gather and thicken at the corrosion interface to improve the corrosion resistance of the steel plate in an acidic environment. These effects are manifested when the content of Cu is above 0.5%. However, when the content of Cu is too high, the corrosion resistance improvement effect is not significant, and the production cost is increased. Therefore, the content of Cu in the steel of the present application is controlled in the range of 0.5-2.00%.

[0026] Cr: Cr in the steel is mostly segregated at the grain boundary, increasing the hardenability of the steel plate and improving the strength and toughness of the steel plate. At the same time, the addition of a small amount of Cr can easily form a passivation film on the surface of the steel plate, effectively delaying the initial corrosion of the steel plate, but when the content of Cr exceeds 0.5%, the corrosion resistance of the steel plate will be reduced with the extension of the corrosion time in the acidic environment. This is mainly because when the segregation concentration of Cr reaches a certain degree, a grain boundary stress layer will be formed, which weakens the corrosion resistance of the grain boundary surface. At the same time, Cr can easily cause a decrease in the C concentration in the pearlite. The present application cooperatively adds Ni and Cu elements to increase the solid solubility of Ni and Cu elements, thereby synergistically playing the roles of Ni, Cu and Cr to increase the corrosion resistance of the microstructure of the steel plate. Therefore, Cr needs to be combined with Cu and Ni to improve the pitting corrosion resistance of the steel plate in an acidic environment. In order to obtain the above effects, the content of Cr in the steel of the present application is controlled in the range of 0.5-2.0%.

[0027] Mo: Mo is an element that effectively improves the hardenability and has stable chemical properties, which can promote the formation of a passivation film of Cr on the surface of the steel plate. In the present application, Mo can form a stable Cl -The dense rust layer of the acid corrosion is resistant to the chloride ion point corrosion. When the Mo content is less than 0.5%, the above effect is not obvious, but when the Mo content is more than 1.50%, the production cost is increased and the toughness is deteriorated, therefore, the Mo content in the steel is controlled in the range of 0.5-1.50%.

[0028] Ti: Ti can fix N element, the stoichiometric ratio of Ti / N is 3.42, about 0.02% Ti can fix N in the steel below 60ppm, and fine high-temperature stable TiN precipitates can be formed during the slab continuous casting. The fine TiN particles can effectively hinder the austenite grain growth during the slab reheating, and help to increase the Nb solid solubility in the austenite when Nb is added in the steel plate, and obviously improve the impact toughness of the welding heat affected zone. However, when the Ti content is too high, coarse nitrides will be formed or TiC will be generated, which reduces the toughness of the base material and the welding heat affected zone. The addition of Ti can also form TiS to avoid the formation of MnS as the corrosion starting point, and improve the point corrosion resistance of the steel plate. Considering comprehensively, the Ti content in the present application is designed to be 0.005-0.15%.

[0029] V: V can play a role in solid solution strengthening in the steel, on the other hand, excessive V will have adverse effects on the toughness and welding of the steel plate, therefore, considering comprehensively, the maximum V added in the present application is 0.15%, which is selectively added according to the process and needs.

[0030] Nb: NbC is strain-induced precipitated during hot rolling to hinder the recovery and recrystallization of the deformed austenite, and the deformed austenite structure in the non-recrystallization zone is converted into fine phase transformation products during phase transformation through controlled rolling and controlled cooling, which has a very obvious effect on grain refinement, so that the steel has high strength and high toughness. Effective grain refinement is beneficial to improving corrosion resistance, when the Nb content is higher than 0.15%, the grain refinement effect is no longer significant, and the toughness is deteriorated, therefore, the Nb content in the steel is controlled to be ≤0.15%.

[0031] Sb: Sb element can significantly improve the corrosion resistance of the steel by inhibiting the anodic and cathodic reactions of the steel in the acid solution. The impedance value of the steel increases after the addition of antimony, thereby enhancing the corrosion resistance of the steel in the acid medium, but excessive Sb will significantly deteriorate the toughness, therefore, the Sb content is controlled to be ≤0.15% in the present application, and preferably 0.012%≤Sb≤0.15%.

[0032] Zr: Zr can combine with S to form sulfides in the steel, reduce the generation of MnS, and improve the point corrosion resistance of the steel plate. A small amount of Zr can produce the above effect, but when the Zr content is more than 0.25%, the toughness of the steel plate will be deteriorated. Therefore, the Zr content in the steel is controlled to be ≤0.25%.

[0033] REM: The addition of REM can improve the morphology of sulfides, and the oxides and sulfides of REM can inhibit the growth of austenite grains during the welding thermal cycle. However, when the content of REM is greater than 0.02%, part of the inclusions with a particle size greater than 5 μm will be generated, which reduces the impact toughness of the base material and the welding heat affected zone. Therefore, the content of REM in the steel of the present application is controlled to be ≤0.02%.

[0034] In addition, the chemical composition of the steel of the present application can ensure to obtain a good matching of strength, toughness and corrosion resistance, which is suitable for the acidic corrosion environment of the inner bottom plate of the cargo oil tank of the crude oil tanker. Ni and Cu elements each act as a solid solution element, and are easy to form a passivation layer on the surface of the steel plate in the acidic environment to slow down the acidic corrosion, at the same time, cooperating with Cr element to reduce the solid solution C content in the matrix structure, which can further increase the synergistic effect of Ni and Cu, form a dense and thick passivation film protective layer, enhance the passivation ability of the structure, and delay the occurrence of pitting corrosion of the steel plate. Therefore, the content of Ni, Cu and Cr satisfies the relationship Cr≥(39+300Cu+590Ni) / 100, in which the weight percentage of each element in the relationship is calculated. For example, assuming that Cu is 1% and Ni is 2%, the relationship should be calculated as (39+300*1%+590*2%) / 100=0.538.

[0035] In addition, the addition of Mo element promotes the formation of Cr passivation layer of iron, which can further play a synergistic effect with Cr, and increase the passivation corrosion resistance of the product by means of its stable compound characteristics. In addition, Ni, Cu, Cr and Mo elements are beneficial to the strength and toughness of the steel plate. Therefore, in order to match the strength and toughness, the addition amount of Mn should not be too much.

[0036] Preferably, the surface of the steel plate also has a passivation film containing the following components in weight percentage: Ni≥5.0%, Cu≥27%, Cr≥3.2%.

[0037] The passivation film can be actively formed after the manufacture of the steel plate, or can be formed by the corrosion of crude oil on the steel plate after the steel plate is installed as a crude oil cargo tank. The method of actively forming a passivation film on the steel plate is well known to those skilled in the art, and will not be described here.

[0038] Here, the meaning of "the surface of the steel plate" includes the area of 100 μm depth vertically downward from the surface of the steel plate.

[0039] In the second aspect, the present application also provides a manufacturing method of the steel plate for the inner bottom plate of the cargo oil tank of the crude oil tanker, which comprises the following steps performed in sequence:

[0040] 1) Smelting and casting

[0041] Based on the above composition, smelting, refining and continuous casting are carried out to form a casting blank;

[0042] 2) Rolling and cooling

[0043] The heating temperature of the casting blank is 1050-1250℃, the surface temperature uniformity of the casting blank is ≤20℃, the holding time is (1.8-2.2)×t min, t is the thickness of the casting blank, and then rough rolling and finish rolling are carried out.

[0044] After rolling, air cooling or water cooling at a cooling speed of 2-6℃ / s is carried out to below 400℃.

[0045] Preferably, the single pass reduction rate of the rough rolling is 8-10%.

[0046] In the rolling and cooling process, the heating temperature before rolling is controlled to be 1050-1250℃. When the heating temperature before rolling is less than 1050℃, the carbonitride of microalloy elements such as Nb and Ti cannot be completely solid-solved. More importantly, in this temperature range, the surface temperature uniformity of the casting blank is controlled to be ≤20℃, and the holding time is kept at (1.8-2.2)×t min, which can ensure the surface temperature uniformity of the steel plate, and control the holding time to promote the best passivation film effect of Cu and Ni elements, so that the passivation film structure is dense. When the heating temperature is greater than 1250℃, the growth of austenite grains will be caused.

[0047] After the finish rolling, slow cooling or air cooling at a cooling speed of 2-6℃ / s is carried out to below the final cooling temperature 400℃, and the high-density dislocations formed by low-temperature rolling are used as nucleation points to form uniform and fine ferrite+small amount of pearlite structure or ferrite+small amount of acicular ferrite structure.

[0048] Preferably, in step 2), the rough rolling temperature is ≥940℃, and the cumulative reduction rate of rough rolling is ≥60%. In the present application, the rough rolling temperature is controlled to be ≥940℃ to better promote the grain boundary segregation of passivation elements, and at the same time, the cumulative reduction rate of rough rolling is required to be ≥60% to make the recrystallization fully occur, refine the austenite grains, and avoid abnormal grain growth to reduce the toughness of the base material.

[0049] Preferably, in step 2), the finish rolling temperature is 720-880℃, and the cumulative reduction rate of finish rolling is ≥60%.

[0050] In the present application, the finish rolling temperature is controlled to be 720-880℃, and the cumulative reduction rate of finish rolling is ≥60%. When the cumulative reduction rate of finish rolling is less than 60%, the steel plate is not deformed enough, the dislocation density formed is less, the nucleation position is reduced, the grain refinement effect of the steel plate is not significant, and the final strength and toughness performance of the steel plate is affected. Therefore, in the present application, the finish rolling temperature is controlled to be 720-880℃, and the cumulative reduction rate of finish rolling is ≥60%.

[0051] Preferably, the rough rolling single pass reduction rate is controlled at 8-10%.

[0052] Preferably, the slab thickness t is 300-450 mm.

[0053] The beneficial effects of the present application are:

[0054] The present application is designed with low C and low Mn, and with the addition of elements such as Ni, Cu, Cr and Mo. The addition of Cr element cooperates with the addition of Ni and Cu elements, which further increases the synergistic effect of Ni and Cu, helps to form a dense and thick passivation film protective layer, enhances the passivation ability of the structure, delays the occurrence of pitting corrosion of the steel plate, and the added Mo and Cr elements form a dense rust layer on the surface of the steel plate. Therefore, in the composition design of the present application, the steel plate can be resistant to Cl - acid environment, and can form a thick and dense passivation film and a dense rust layer on the steel plate corrosion interface in the presence of H2S and SO2 in the crude oil deposition water + elemental corrosion environment, thereby enhancing the Cl - acid resistance of the steel plate. The use of traditional W elements to improve corrosion resistance has a negative impact on the welding performance of the steel plate.

[0055] On the basis of the composition design, the present application further utilizes large reduction process to fully refine the grain structure, ensures the strength and toughness of the steel plate, controls the uniformity of the slab surface temperature and the holding time to control the aggregation of micro precipitates at the grain boundary, and strengthens the corrosion resistance of the grain boundary. The rolling conditions are controlled to obtain a microstructure of ferrite + a small amount of pearlite or ferrite + a small amount of acicular ferrite, wherein the area ratio of ferrite structure FA% is ≥ 90%, and the average grain size of ferrite d is 5-15 μm, which significantly improves the corrosion resistance of the steel plate in the crude oil tanker cargo tank bottom plate corrosion environment.

[0056] The yield strength of the steel plate manufactured by the present application is ≥ 355 MPa, the tensile strength is 500-700 MPa, the elongation is ≥ 35%, and the impact energy at -60 ℃ is ≥ 300 J. The impact toughness at low temperature is excellent. In the simulated crude oil tanker cargo tank strong acid corrosion test environment with a PH value of 0.85, the corrosion resistance of the crude oil tanker cargo tank bottom plate provided by the present application is 5 times that of ordinary steel plate, and the average corrosion rate CR is ≤ 0.8 mm / year. It can be used to manufacture large crude oil tanker cargo tank bottom plate structures and other marine and ship corrosion-resistant structures. BRIEF DESCRIPTION OF DRAWINGS

[0057] Fig. 1 is a schematic view of the size of the base material sample in the corrosion resistance detection of the embodiment of the present application.

[0058] Fig. 2 is a schematic view of the size of the welded joint sample in the corrosion resistance detection of the embodiment of the present application.

[0059] Figure 3 is a schematic diagram of the corrosion test in the corrosion resistance detection of the embodiment of the present application. DETAILED DESCRIPTION

[0060] The present application is further described below in conjunction with embodiments and the accompanying drawings.

[0061] The components of the example steels and the comparative steels shown in Table 1 were smelted and cast to form a casting blank, and then rolled and cooled according to the manufacturing process parameters of the example steels and the comparative steels of the present application shown in Table 2. Then, the mechanical properties and corrosion resistance of the example steels and the comparative steels of the present application were tested, as shown in Table 3.

[0062] The mechanical properties of the steel plate of the present application were determined according to GB / T228 and GB / T229.

[0063] The microstructure of the steel plate of the present application was determined by GB / T15124 and GB / T6394.

[0064] The sampling and testing process for the specific corrosion resistance of the steel plate of the present application is as follows:

[0065] (1) The corrosion sample was taken at the thickness of 1 / 4 of the steel plate, and the size of each sample was 25±1mm x 60±1mm x 5±0.5mm. The size of the base material sample is shown in Figure 1.

[0066] The corrosion sample of the base plate corrosion-resistant steel welded joint was 25±1mm x 60±1mm x 5±0.5mm in size, which included a weld metal with a width of 15±5mm. The size of the welded joint sample is shown in Figure 2. Except for the hole used for hanging, the surface of the sample was polished with 600# sandpaper.

[0067] (2) To prevent cracks and / or local corrosion, the sample was hung in the solution with fine nylon (diameter 0.3mm to 0.4mm). The corrosion test schematic diagram is shown in Figure 3.

[0068] (3) The test solution contained 10% by weight of sodium chloride, and the pH value of the solution was adjusted to 0.85 with hydrochloric acid solution. To reduce the change of the pH value of the test solution, the test solution was replaced every 24 hours. The volume of the solution was greater than 20cc / cm 2 (the surface area of the sample), and the temperature of the test solution should be maintained at 30±2℃.

[0069] (4) The size and original weight of the sample were recorded before the experiment, and the weight of the sample after the test was recorded after the experiment to calculate the weight loss.

[0070] Corrosion Rate is calculated according to the following formula: CR (mm / year) = 365 (days) x 24 (hours) x W x 10 / (S x 72 (hours) x D), wherein W is weight loss (g), D is density of the sample (g / cm 3 ), and S is surface area of the sample (cm 2 ).

[0071] For the sample with cracks and / or local corrosion, the CR value is plotted on a standard normal distribution statistical chart, and if the value deviates from the normal statistical distribution state, the test result should be rejected, and finally the average value of the corrosion rate is calculated according to the effective data points.

[0072] After corrosion, the surface of the steel plate is cleaned with alcohol, and then element depth analysis is performed by using a glow discharge optical emission spectrometer GDOES, wherein the element enrichment amount of Cu, Ni and Cr in the surface 0-100 μm is shown in Table 3, and the higher the element enrichment concentration, the more dense the passivation film formed, and it can be known that the improvement of the corrosion resistance of the steel plate is closely related to the element enrichment degree of Cu, Ni and Cr on the surface.

[0073] As shown in Table 3, the yield strength (Rp0.2) of the steel plate manufactured by the application is greater than or equal to 355 MPa, the tensile strength (Rm) is 500-700 MPa, the impact energy at -60 ℃ (vE-60) is greater than or equal to 300 J, the elongation (A) is greater than or equal to 35%, the area ratio of ferrite structure in the microstructure is greater than or equal to 90%, the average grain size of ferrite is 5-15 μm, the corrosion resistance in the corrosion test environment is that the annual average corrosion rate CR is less than or equal to 0.8 mm / year, and the steel plate can be used to manufacture large crude oil tanker cargo tank bottom plate structure and other marine and ship corrosion-resistant structures.

Claims

1. A steel plate for crude oil tanker cargo tank bottom, said steel plate comprising, in addition to Fe and unavoidable impurities, the following components in weight percent: C: 0.010-0.10%, Si: 0.10-0.50%, Mn: 0.5-1.5%, P: 0.008-0.060%, S≤0.010%, Cr: 0.5-2.0%, Mo: 0.5-1.50%, Cu: 0.5-2.00%, Ni: 0.5-2.50%, Al: 0.01-0.12%, Ti: 0.005-0.15%; and satisfying Cr≥(39+300Cu+590Ni) / 100, wherein each symbol represents the weight percent of the corresponding element.

2. The steel plate for a crude oil tanker cargo tank bottom plate according to claim 1, characterized by, The steel plate further comprises at least one of the following: V≤0.15%, preferably 0.04%≤V≤0.15%, Nb≤0.15%, preferably 0.012%≤Nb≤0.15%, Sb≤0.15%, preferably 0.012%≤Sb≤0.15%, N≤0.010%, Zr≤0.25%, preferably 0.01%≤Zr≤0.25%, REM≤0.020%, preferably 0.002%≤REM≤0.02%.

3. The steel plate for a crude oil tanker cargo tank bottom plate according to claim 1, characterized by, The steel plate comprises, in weight percent: C: 0.010-0.10%, Si: 0.10-0.50%, Mn: 0.5-1.5%, P: 0.008-0.060%, S≤0.010%, Cr: 0.5-2.0%, Mo: 0.5-1.50%, Cu: 0.5-2.00%, Ni: 0.5-2.50%, Al: 0.01-0.12%, Ti: 0.005-0.15%, and the balance of Fe and unavoidable impurities.

4. The steel plate for a crude oil tanker cargo tank bottom plate according to any one of claims 1 to 3, characterized by, The steel plate has a microstructure of ferrite + pearlite or ferrite + acicular ferrite, wherein the area ratio of ferrite microstructure is≥90%, and the average grain size of ferrite is 5-15 μm.

5. The steel plate for a crude oil tanker cargo tank bottom plate according to any one of claims 1 to 3, characterized by, The steel plate further has a passivation film on the surface, said passivation film comprising, in weight percent: Ni≥5.0%, Cu≥27%, Cr≥3.2%.

6. The steel plate for a crude oil tanker cargo tank bottom plate according to any one of claims 1 to 3, characterized by, The steel plate has a yield strength≥355 MPa, preferably≥420 MPa, a tensile strength of 500-700 MPa, an elongation≥35%, an impact energy at -60°C≥300 J, and an average corrosion rate CR≤0.8 mm / year.

7. A method of manufacturing the steel plate for a crude oil tanker cargo tank bottom plate according to any one of claims 1 to 6, characterized by, The method comprises the following steps in sequence: 1) Smelting and casting Smelting, refining and continuous casting are performed based on the steel plate composition of any one of claims 1-3 to form a cast slab; 2) Rolling and cooling The heating temperature of the cast slab is 1050-1250°C, the surface temperature uniformity of the cast slab is≤20°C, and the holding time is (1.8-2.2)xt, unit: min, wherein t is the thickness of the cast slab, unit: mm; then rough rolling and finish rolling are performed; After rolling, air cooling or water cooling at a cooling rate of 2-6°C / s is performed to below 400°C.

8. The method of claim 7, wherein, The single pass reduction rate of the rough rolling is 8-10%.

9. The method of claim 7, wherein, In step 2), the rough rolling temperature is ≥ 940℃, and the cumulative reduction ratio of rough rolling is ≥ 60%.

10. The method of claim 7, wherein, In step 2), the finish rolling temperature is 720-880℃, and the cumulative reduction ratio of finish rolling is ≥ 60%.

11. The method of claim 7, wherein, The slab thickness t is 220-450 mm.

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