High strength pipe resistant to sodium hydroxide corrosion and method for manufacturing same

Abstract

The present invention discloses a pipe, the pipe having a corrosion-resistant layer and a base layer in the thickness direction, the corrosion-resistant layer being disposed at least on the inner wall of the pipe, and the corrosion-resistant layer containing, in addition to Fe and inevitable impurities, further elements in the following weight percentages: 0 < C ≤ 0.05%; Si: 0.3 - 0.6%; Mn: 0.5 - 2.0%; Ni: 8.00 - 14.00%; Cr: 16.00 - 19.00%; Mo: 2.00 - 3.50%; N: 0.02 - 0.20%; and Ti: 0.01 - 0.2%, and Cr, Mo, N, and Ti satisfy the following inequality: Cr + 2.8×Mo + 16×N + 2×Ti ≥ 22.0%. Correspondingly, the present invention further discloses a manufacturing method of the above pipe, including the following steps: (1) a step of preparing a corrosion-resistant layer slab and a base layer slab; (2) a step of assembling the corrosion-resistant layer slab and the base layer slab to obtain a composite slab; (3) a hot rolling step: a step of heating the composite slab at a temperature of 1150 - 1230°C, wherein the total rolling reduction ratio is 90% or more and the finishing rolling temperature is 900°C or more; (4) a winding step: after water cooling, winding at a temperature of 650 - 700°C to obtain a hot-rolled coil; (5) a step of subjecting the hot-rolled coil to surface treatment; and (6) a step of manufacturing the pipe.

Description

[Technical field]

[0001] The present invention relates to a pipe and a method for manufacturing the same, and in particular to a sodium hydroxide corrosion-resistant high-strength pipe and a method for manufacturing the same. [Background technology]

[0002] As is well known in the art, in the current water supply plant site, in order to ensure the water quality, the membrane pool alkaline cleaning process is usually used for treatment. In the actual application process, the injection pipeline, the flushing pipeline and the pipeline from the membrane pool to the neutralization pool, which are used in the membrane pool alkaline cleaning process, are easily corroded by sodium hydroxide. Therefore, in order to prevent these pipelines from being corroded by sodium hydroxide as much as possible, 304 and 316L stainless steel materials are usually used to manufacture such pipelines.

[0003] Due to this requirement, the above pipelines and equipment cannot be made of carbon steel pipes, which will inevitably be corroded by sodium hydroxide, resulting in water pollution and even pipeline leakage and pipeline rupture.

[0004] Compared with carbon steel, the structure of 304 and 316L stainless steel has low strength. Under the same working conditions, the required thickness is higher, resulting in more material usage; At the same time, welding and machining of 304 and 316L stainless steel is more difficult; In addition, 304 and 316L stainless steel itself contains many precious metal elements such as Cr, Ni, Mo, etc., which results in the relatively high cost of pure stainless steel in the process of production, manufacturing and installation.

[0005] Based on the above, and taking into consideration the shortcomings and defects of the above 304 and 316L stainless steels and carbon steels in the application to sodium hydroxide corrosion environment, the present invention intends to obtain a new pipe, which adopts a reasonable design, so that a corrosion-resistant layer resistant to sodium hydroxide corrosion can be formed on the surface of the carbon steel base layer, and finally forms a pipe with sodium hydroxide corrosion resistance, good mechanical properties, and high economic efficiency, thereby achieving great economic and social benefits.

[0006] Summary of the Invention One of the objectives of the present invention is to provide a new pipe, which adopts a rational design, so that a corrosion-resistant layer resistant to sodium hydroxide corrosion can be formed on the surface of a carbon steel base layer, and finally a pipe with sodium hydroxide corrosion resistance, good mechanical properties, and high economic efficiency can be formed.

[0007] The pipe described in the present invention solves the essential defects of 304, 316L stainless steel or carbon steel occurring in the sodium hydroxide corrosive medium environment in the current water supply plant field, meets the ever-increasing requirements for the corrosion resistance and mechanical properties of the pipe of the pipeline equipment in the sodium hydroxide medium environment in the water supply plant field, and greatly improves the applicability, safety and durability of the pipeline equipment while avoiding secondary pollution of water quality, thereby achieving great economic and social benefits.

[0008] To achieve the above object, the present invention provides a pipe, the pipe having a corrosion-resistant layer and a base layer in a thickness direction, the corrosion-resistant layer being arranged at least on an inner wall of the pipe, the corrosion-resistant layer further comprising the following chemical elements in weight percentages in addition to Fe and unavoidable impurities: 0 <C≦0.05%; Si: 0.3-0.6%; Mn: 0.5-2.0%; Ni: 8.00~14.00%; Cr: 16.00~19.00%; Mo: 2.00-3.50%; N: 0.02-0.20%; and Ti: 0.01~0.2%; Here, Cr, Mo, N and Ti satisfy the following inequality: Cr+2.8×Mo+16×N+2×Ti≧22.0%.

[0009] Preferably, in the pipe according to the invention, the corrosion resistant layer consists of the following chemical elements in the following weight percentages: 0 <C≦0.05%; Si: 0.3-0.6%; Mn: 0.5-2.0%; Ni: 8.00~14.00%; Cr: 16.00~19.00%; Mo: 2.00-3.50%; N: 0.02-0.20%; Ti: 0.01-0.2%; and Residual Fe and unavoidable impurities; Here, Cr, Mo, N and Ti satisfy the following inequality: Cr+2.8×Mo+16×N+2×Ti≧22.0%.

[0010] In the pipe according to the present invention, in order to ensure the sodium hydroxide corrosion resistance of the pipe, the sodium hydroxide corrosion resistance of the corrosion-resistant layer should be ensured first.

[0011] At present, the working conditions of sodium hydroxide corrosive medium at the existing water supply plant site are: a transport medium containing sodium hydroxide at a mass concentration of 0-30%, the temperature of which is between 0-40°C.

[0012] Based on the above working conditions, the inventors optimize the chemical element composition of the corrosion-resistant layer in the pipe, especially add a certain amount of Ti element, and design a specific corrosion resistance equivalent formula and requirement, such that Cr, Mo, N and Ti are controlled to satisfy the formula: Cr+2.8×Mo+16×N+2×Ti≧22.0%, thereby further reducing the susceptibility to intergranular corrosion, preventing the stress corrosion of pipeline equipment in the alkaline cleaning process, and avoiding the corrosion of the welded seam caused by sodium hydroxide solution when the temperature is further increased.

[0013] In the present invention, the design principles of each chemical element in the corrosion-resistant layer are as follows:

[0014] C: In the corrosion-resistant layer described in the present invention, C is a strong austenite-forming element, which can replace nickel to a certain extent, promote austenite formation, and stabilize the austenite structure while increasing the strength of stainless steel. However, the content of C element should not be too high. If the carbon content is too high, chromium-rich carbides will be formed at the grain boundaries after carbon combines with chromium, which will cause intergranular corrosion. Therefore, in order to bring out the beneficial effect of C element, in the corrosion-resistant layer described in the present invention, the mass percentage of C element can be controlled to satisfy the following: 0.05% by mass of C element; ... <C≦0.05%。

[0015] Si: In the corrosion-resistant layer described in the present invention, it is typically necessary to add a certain amount of silicon, since the Si element is mainly used for deoxidation during the smelting process. However, it should be noted that the Si element content should not be too high. If the Si content is too high, the solubility of nitrogen will decrease. Therefore, in the corrosion-resistant layer described in the present invention, the mass percentage of the Si element can be controlled between 0.3-0.6%.

[0016] Mn: In the corrosion-resistant layer described in the present invention, Mn element is an element that strongly stabilizes the austenite structure and can increase the solubility of nitrogen in steel. However, manganese also has a negative effect on the corrosion resistance of austenitic stainless steel at the same time. Therefore, considering the beneficial and harmful effects of Mn element, in the corrosion-resistant layer described in the present invention, the mass percentage of Mn element can be controlled between 0.5-2.0%, preferably 1.0-2.0%, more preferably 1.0-1.36%.

[0017] Ni: In the corrosion-resistant layer described in the present invention, Ni is the most important element for forming and stabilizing the austenite phase. Adding an appropriate amount of Ni element can ensure the formation of the austenite structure of the steel at room temperature. However, since the price of nickel is expensive, it is not appropriate to add excessive nickel to the steel in order to ensure a relatively low cost. Therefore, in the corrosion-resistant layer described in the present invention, the mass percentage of Ni element can be controlled between 8.00-14.00%, preferably 8.00-10.00%.

[0018] Cr: In the corrosion-resistant layer described in the present invention, Cr is the guarantee for obtaining the rust resistance and corrosion resistance of stainless steel. Generally, the minimum chromium content for obtaining corrosion resistance is 10.5%. Since chromium is an element that significantly increases corrosion resistance, in order to ensure good corrosion resistance, the chromium content in the steel of the present invention is controlled to 16.0% or more. However, it should be noted that chromium is also the main ferrite-forming element, and if the chromium content is too high, it is difficult to ensure that the corrosion-resistant layer obtains an austenitic structure at room temperature. Therefore, in the corrosion-resistant layer described in the present invention, the mass percentage of Cr element can be controlled between 16.00 and 19.00%.

[0019] Mo: In the corrosion-resistant layer described in the present invention, Mo is an important element for improving corrosion resistance, and its mechanism is to stabilize the passive film and promote the enrichment of chromium in the passive film; in addition, molybdenum can also cooperate with nitrogen to further improve pitting corrosion resistance, so the main effect of adding molybdenum is to improve corrosion resistance. However, the content of Mo element should not be too high. If the content of molybdenum is too high, the cost of the alloy will increase. Therefore, in order to ensure a relatively low cost, in the corrosion-resistant layer described in the present invention, the mass percentage of Mo element can be controlled between 2.00-3.50%, preferably 2.00-3.00%.

[0020] N: In the corrosion-resistant layer described in the present invention, N is an element that forms, stabilizes and expands the austenite zone very strongly. By adding an appropriate amount of N element, the pitting corrosion resistance of stainless steel can be effectively improved. However, if the nitrogen content in steel is too high, the risk of forming nitrogen-containing intermetallic phase increases, and at the same time, the difficulty of smelting and hot working increases, leading to manufacturing difficulties. Therefore, in the corrosion-resistant layer described in the present invention, the mass percentage of N element can be controlled between 0.02-0.20%, preferably 0.02-0.10%.

[0021] Ti: In the corrosion-resistant layer described in the present invention, Ti is an element that stabilizes titanium compounds and nitrogen compounds. By adding an appropriate amount of Ti element to steel, the decrease in corrosion resistance caused by the decrease in chromium concentration due to the formation of compounds between chromium and carbon can be effectively prevented; in addition, by adding an appropriate amount of Ti element, pitting corrosion caused by MnS can also be prevented. However, the Ti element content in steel should not be too high. If the Ti element content in steel is too high, not only will the risk of formation of intermetallic phases containing titanium and nitrogen increase, but it will also reduce the thermoplasticity of steel, increase the difficulty of smelting and hot working, and lead to manufacturing difficulties. Therefore, in the corrosion-resistant layer described in the present invention, the mass percentage of Ti element can be controlled between 0.01-0.2%, preferably 0.1-0.2%.

[0022] Correspondingly, in the corrosion-resistant layer of the pipe described in the present invention, the mass percentage of only a single chemical element is controlled, while the elements Cr, Mo, N and Ti are further controlled to satisfy the formula: Cr+2.8×Mo+16×N+2×Ti≧22.0%, where each element is replaced by the mass percentage of the corresponding element. Through such control, it can be further ensured that the corrosion-resistant layer has rather excellent corrosion resistance in sodium hydroxide environment.

[0023] Preferably, in the pipe according to the invention, in the corrosion resistant layer, the unavoidable impurities include: S≦0.030%; P≦0.045%.

[0024] In the corrosion-resistant layer of the pipe described in the present invention, both P and S are unavoidable impurity elements. In order to ensure the quality of the steel, it is preferable that the content of impurity elements in the steel is as low as possible if conditions permit.

[0025] Preferably, in the pipe according to the invention, the base layer comprises the following weight percentages of chemical elements: C: 0.01-0.20%; Si: 0.10-0.30%; Mn: 0.50-1.50%; Al: 0.02-0.03%; Ti: 0.005~0.018%; Nb: 0.005~0.020%; N ≤ 0.006%; and Residual Fe and unavoidable impurities.

[0026] Preferably, in the pipe according to the invention, the base layer further comprises at least one of the following chemical elements: 0 <B≦0.0003%; 0 <Ni≦0.20%; 0 <Cr≦0.20%; 0 <Mo≦0.10%。

[0027] In the present invention, in order to ensure that the pipe described in the present invention can obtain higher mechanical properties, the selection of the chemical element composition of the carbon steel in the base layer needs to ensure both high strength and good workability.

[0028] Therefore, in the present invention, the design principles of each chemical element of carbon steel in the base layer are as follows:

[0029] C: In the base layer described in the present invention, C is an element that stabilizes austenite, and can achieve the effect of solid solution strengthening in steel, and can obviously improve the strength of steel. At the same time, the content of C element in steel should not be too high. If the content of C element in steel is too high, it is not only disadvantageous to the welding performance and toughness of the material, but also prone to increase hard phase structures such as pearlite structure and martensite-austenite island, which adversely affect the corrosion resistance of steel. Therefore, considering the requirements for the strength and toughness alignment of steel plate and the corrosion resistance of carbon steel material, in the base steel material described in the present invention, the mass percentage of C element can be controlled between 0.01-0.20%, preferably 0.10-0.20%.

[0030] Si: In the base layer described in the present invention, the purity and deoxidation of steel can be effectively increased by adding Si element to steel. Although Si element can provide the effect of solid solution strengthening to steel, Si element does not contribute to the welding performance of the material; in the present invention, the silicon content of carbon steel in the base layer is controlled to 0.3% or less, which does not affect the corrosion resistance of the corrosion-resistant layer, and does not have any adverse effect on the corrosion resistance of the corrosion-resistant layer, but in such a case, the carbon steel in the base layer has good welding performance. Therefore, in the base layer described in the present invention, the mass percentage of Si element can be controlled between 0.10% and 0.30%.

[0031] Mn: In the base layer described in the present invention, the addition of Mn element to the steel can delay the pearlite transformation, reduce the critical cooling rate, and improve the hardenability of the steel; at the same time, Mn element, which is the main solid solution strengthening element in steel, also has the effect of solid solution strengthening. However, it should be noted that Mn element should not be added in large amounts. If the Mn element content in the steel is too high, segregation bands and martensite structures are likely to appear, which adversely affects the toughness of the steel, and the appearance of segregation bands also reduces the corrosion resistance of the steel. The amount of Mn added depends mainly on the strength level of the steel. Generally, the manganese content in low carbon microalloy steel does not exceed 1.50%. At the same time, the Mn element contained in the carbon steel of the base layer does not have a detrimental effect on the corrosion-resistant layer. Therefore, in the base layer described in the present invention, the mass percentage of Mn element can be controlled between 0.50-1.50%, preferably 1.00-1.50%.

[0032] Al: In the base layer described in the present invention, Al is a strong deoxidizing element. Al is added to ensure that the oxygen content in the steel is as low as possible. After deoxidization, the excess Al and N elements in the steel can combine to form AlN precipitates, which can improve the strength of the steel and refine the austenite grain size of the steel during heat treatment. However, it should be noted that if the Al content is too high, Al oxide-based inclusion defects are likely to occur. Therefore, in the base layer described in the present invention, the mass percentage of Al element can be controlled between 0.02-0.03%.

[0033] Ti: In the base layer described in the present invention, Ti is a strong carbide-forming element. Adding a small amount of Ti to steel helps to fix N in the steel. TiN formed by the combination of Ti and N can prevent excessive growth of austenite grains during heating of the slab material in the base layer, thereby refining the initial austenite grains. In addition, titanium in steel can also react with carbon and sulfur to form substances such as TiC, TiS and Ti4C2S2, respectively, which exist in the form of inclusions and second phase particles. Furthermore, these titanium carbonitride precipitates can prevent grain growth in the heat-affected zone during welding, thereby improving the welding performance. However, it should be noted that too high Ti content can lead to an increase in the number of carbides such as titanium carbide, and their segregation and enrichment, which affects the mechanical properties of the material. Therefore, in the base layer described in the present invention, the mass percentage of Ti element can be controlled between 0.005-0.018%.

[0034] Nb: In the base layer described in the present invention, Nb is a strong carbide-forming element. A small amount of Nb is added to the steel mainly to increase the recrystallization temperature to match the high finish rolling temperature of the composite plate formed by the assembly between the base layer slab and the corrosion-resistant layer slab during the subsequent rolling process, so that the grains are refined after the rolling in the recrystallized and non-recrystallized regions is completed, thereby improving the low-temperature impact toughness of the base layer. However, it should be noted that if the Nb content is too high, it is easy to generate large Nb-rich precipitates, which results in a decrease in the plasticity and toughness of the material. Therefore, in the base layer described in the present invention, the mass percentage of Nb element can be controlled between 0.005-0.020%.

[0035] N: In the base layer described in the present invention, N element can combine with titanium and aluminum to form second phase particles, which refine the grains and improve the strength. However, if the mass percentage of N element is too high, the amount of TiN obtained will be too large, which will cause the particles to become coarse, affecting the plasticity of the base carbon plate of the composite material described in the present invention. Therefore, in the base layer described in the present invention, the mass percentage of N element can be controlled to be N≦0.006%.

[0036] B: In the present invention, B can significantly improve the hardenability of steel. When a highly corrosion-resistant composite plate (corrosion-resistant layer slab + base layer slab) is subjected to air cooling after rolling, it is necessary to obtain a ferrite + pearlite structure as much as possible in order to suppress the formation of bainite, especially in highly corrosion-resistant thin steel strips in which the thickness of a single layer of the corrosion-resistant layer accounts for 0.5% to 20% of the total thickness. However, it should be noted that if the B content is too high, segregation will easily occur. Therefore, in the base layer described in the present invention, the amount of B element added is 0. <B≦0.0003%であってもよい。

[0037] Ni: In the present invention, Ni is an element that stabilizes austenite and has a certain effect on improving the strength of steel. In addition, by adding an appropriate amount of Ni to steel, especially by adding an appropriate amount of Ni to quenched and tempered steel, the low-temperature impact toughness of the steel can be significantly improved. However, if the Ni content is too high, the carbon equivalent will increase, thereby affecting the welding performance; at the same time, it should be noted that Ni is a rare metal, which will result in resource waste and increased costs. Therefore, in the base layer described in the present invention, the amount of Ni element added can be controlled to satisfy the following: 0 <Ni≦0.20%。

[0038] Cr: In the present invention, the segregation tendency of the Cr element is smaller than that of Mn. When the Mn content in the carbon steel base layer is high, and when there are obvious segregation bands and strip-like structures in the steel, the Mn content can be appropriately reduced, and insufficient parts can also be replaced with Cr. Secondly, by adding an appropriate amount of Cr element to the carbon steel in the base layer, it is also beneficial to inhibit the diffusion of Cr in the corrosion-resistant layer into the base layer. However, it should be noted that if the Cr content is too high, it will affect the phase transition temperature of the base material and the mechanical properties of the base material. Therefore, in the base layer described in the present invention, the Cr element can be added in the range of 0 < Cr ≤ 0.20%.

[0039] Mo: In the present invention, the Mo element can significantly refine the grains and improve the strength and toughness of the steel. Also, Mo can reduce the temper brittleness of the steel, and at the same time, very fine carbides can be precipitated during tempering, thereby significantly strengthening the matrix of the steel. In addition, the addition of the Mo element is also beneficial to suppressing the self-temper brittleness of the steel plate that is likely to occur during the air-cooling process. However, it should be noted that if the Mo content is too high, the carbon equivalent will increase, affecting the welding performance and resulting in waste of resources and cost increase. Therefore, in the base layer described in the present invention, the amount of Mo element added can be controlled to satisfy the following: 0 < Mo ≤ 0.10%.

[0040] Preferably, in the pipe described in the present invention, the base layer contains the following inevitable impurities: S ≤ 0.010%; P ≤ 0.015%.

[0041] In the above technical solution of the present invention, both P and S are inevitable impurity elements in the base layer. S combines with Mn in the steel to form a plastic inclusion, namely manganese sulfide, which is particularly disadvantageous to the plasticity and toughness of the steel in the transverse direction. Therefore, the content of the S element in the base layer should be as low as possible. Also, P is a harmful element in the steel and significantly impairs the plasticity and toughness of the steel plate.

[0042] Therefore, for the present invention, both S and P are unavoidable impurity elements, and their contents should be as low as possible. Considering the actual steelmaking level of a steel plant, in the base layer described in the present invention, S and P elements can be controlled to satisfy: S≦0.010%; P≦0.015%.

[0043] Preferably, in the pipe described in the present invention, the thickness of the single corrosion-resistant layer occupies 0.5 to 20% of the total thickness of the pipe, and more preferably, the thickness of the single corrosion-resistant layer occupies 1.2% to 16.7% of the total thickness of the pipe. That is, when both the inner and outer surfaces of the pipe have corrosion-resistant layers, each corrosion-resistant layer has a thickness that occupies 0.5 to 20% of the total thickness of the pipe. When only the inner surface of the pipe has a corrosion-resistant layer, the corrosion-resistant layer has a thickness that occupies 0.5 to 20% of the total thickness of the pipe.

[0044] In the present invention, the thickness of the corrosion-resistant layer is selected according to the working conditions and plays an important role in obtaining good corrosion resistance, mechanical properties and formability of the pipe described in the present invention. If the corrosion-resistant layer is too thick, it will have a negative effect on the mechanical properties and production cost of the material; if the corrosion-resistant layer is too thin, the corrosion resistance and service life of the material will be reduced. Therefore, in the present invention, preferably, the thickness of the single-layer corrosion-resistant layer is controlled to account for 0.5-20% of the total thickness of the pipe, and more preferably, the single-layer corrosion-resistant layer has a thickness that accounts for 1.2-16.7% of the total thickness of the pipe.

[0045] Preferably, in the pipe according to the present invention, the base layer has a ferrite+pearlite, or ferrite+pearlite+bainite microstructure; and the corrosion resistant layer has an austenite microstructure.

[0046] Preferably, in the pipe according to the present invention, the pipe has a yield strength ≧426 MPa, a tensile strength ≧585 MPa, and an elongation rate ≧35%, and the corrosion-resistant layer has a uniform corrosion rate (i.e., average corrosion rate) ≦0.05 mm / year in an environment with a temperature ≦40° C. and a sodium hydroxide concentration ≦30 wt %.

[0047] Correspondingly, another object of the present invention is to provide a method for manufacturing said pipe, which is simple, feasible and capable of effectively manufacturing said pipe.

[0048] In order to achieve the above object, the present invention provides a method for manufacturing the above pipe, the method comprising the steps of: (1) providing a slab for a corrosion-resistant layer and a slab for a base layer; (2) A step of assembling the corrosion-resistant layer slab and the base layer slab to obtain a composite slab, wherein the thickness of the single layer of the corrosion-resistant layer is preferably 0.5-20% of the total thickness of the composite slab, and more preferably, the thickness of the single layer of the corrosion-resistant layer is 1.2-16.7% of the total thickness of the pipe (the ratio of the thickness of the single layer of the corrosion-resistant layer / the total thickness of the pipe in the completed pipe is substantially the same as the thickness ratio in this step). (3) hot rolling step: heating the composite material at a temperature of 1150 to 1230°C, followed by rolling in multiple passes with a total rolling reduction of 90% or more and a finish rolling temperature of 900°C or more; (4) Coiling process: after water cooling, coiling at a temperature of 650 to 700°C to obtain a hot-rolled coil; (5) subjecting the hot rolled coil to a surface treatment; and (6) Pipe manufacturing process.

[0049] In the above manufacturing method described in the present invention, a high corrosion-resistant composite plate (corrosion-resistant layer plate + base layer plate) can be obtained by the following method: smelting and casting are designed according to the chemical composition to prepare a corrosion-resistant layer slab and a base layer slab, and then the two slabs are assembled. The obtained high corrosion-resistant composite plate is further heated, rolled and coiled to obtain a hot-rolled coil with a composite interlayer structure. After the hot-rolled coil is subjected to surface treatment, pipe fabrication is performed to obtain the pipeline described in the present invention.

[0050] In the present invention, the inventors optimize the heating and rolling process in step (3) to ensure that a transition layer structure with a certain thickness can be formed between the corrosion-resistant layer and the base layer by heating, rolling, etc., so that a perfect metallurgical bond between the corrosion-resistant layer and the base layer can be realized, thereby improving the applicability and economy of the material while ensuring the sodium hydroxide corrosion resistance and mechanical properties.

[0051] In step (2) of the manufacturing method described in the present invention, the prepared corrosion-resistant layer slab and base slab may be pretreated, the joint surfaces of the slabs are sealed by full-circumference welding, and the welded and sealed joint surfaces are evacuated to complete the slab assembly.

[0052] Correspondingly, in step (5), surface treatment can be performed on the hot rolled coil to remove phosphorus using pickling or mechanical methods, and in the pipe fabrication process in step (6), the pipe can be formed and welded using the conventional methods for spiral welded pipe or longitudinal welded pipe. The welding method can be selected from submerged arc welding, gas metal arc welding, tungsten inert gas welding, plasma arc welding, shielded metal arc welding, high frequency welding or laser welding.

[0053] Preferably, in the manufacturing method described in the present invention, the finish rolling temperature is controlled to 920 to 1050°C in the step (3).

[0054] Preferably, in the production method described in the present invention, the method further comprises a preheating step between step (2) and step (3), and the preheating temperature is 1150 to 1230°C.

[0055] In the manufacturing method described in the present invention, whether to select a preheating process between step (2) and step (3) can also be determined according to the performance requirements of the final product. In the preheating process, the composite slab obtained by slab assembly is heated at a temperature of 1150-1230°C, so that the corrosion-resistant layer on the surface of the composite slab can obtain a uniform austenitic structure; so that the carbides that may originally exist are dissolved as completely as possible, while the compounds of alloying elements such as niobium and titanium in the carbon steel are dissolved completely or partially; so that the elements of the corrosion-resistant layer and the carbon steel base layer diffuse at the interface to form a stable transition layer, and then slowly cooled to room temperature.

[0056] Preferably, in the manufacturing method described in the present invention, the method further comprises a cold rolling and annealing step between step (5) and step (6), preferably at an annealing temperature of 900-1000°C.

[0057] In the manufacturing method of the present invention, in practical use, if the target product before pipe production is a cold-rolled coil other than a hot-rolled coil, the method can further include a cold rolling and annealing step between step (5) and step (6). The hot-rolled coil can be cold-rolled to a target thickness and then subjected to annealing.

[0058] As can be seen from the above, in the present invention, the inventors design the composition of the corrosion-resistant layer and the base layer, design the compounding ratio of the two, and use a rolling process to form a corrosion-resistant layer resistant to sodium hydroxide on the surface of the carbon steel plate of the base layer, so as to finally obtain a plate strip with sodium hydroxide corrosion resistance, good mechanical properties and high economic efficiency, and the plate strip can be processed into a pipe for a pipeline device used in the environment of sodium hydroxide corrosive medium at the site of the water supply plant.

[0059] In the design process of the present invention, it should be noted that the following difficulties are involved:

[0060] 1) The design of the composition of the corrosion-resistant layer and the base layer must meet the comprehensive performance of the material. The composition of the corrosion-resistant layer must be designed according to the characteristics of sodium hydroxide corrosion so as to meet the requirements of corrosion resistance under the conditions of use. When designing the carbon steel composition of the base layer, in addition to meeting the requirements of mechanical properties, it is also necessary to consider that when the carbon content of the transition layer obtained by bonding the base layer and the corrosion-resistant layer is relatively high, the lack of stable elements will cause an obvious decarburization layer on the carbon steel side at the interface bond, resulting in a heterogeneous structure of the carbon steel base material, which is easy to form surface defects after processing, and makes it difficult to meet the mechanical performance requirements.

[0061] 2) The difference in metal ratio and material properties of the corrosion-resistant layer and the base layer makes it difficult to control the heating process, rolling process and heat treatment process. For example, if the temperature is not uniform during the heating process, it will lead to deformation and expansion. As a result, it makes it impossible to bond the corrosion-resistant layer to the base layer. For example, the corrosion-resistant layer is easy to peel off from the base layer and crack during the rolling process. As a result, it is difficult to ensure the uniformity of the thickness.

[0062] 3) In the pipeline equipment used in the sodium hydroxide corrosive medium environment at the water supply plant site, such as the injection pipeline in the alkaline cleaning process of the membrane pool, the flushing pipeline, and the pipeline from the membrane pool to the neutralization pool, if the initial composite is not completed and the joint quality is not good, the continuity and uniformity of the corrosion-resistant layer of the completed pipeline after subsequent pickling, pipe forming, welding and other processes cannot be guaranteed, which will cause great potential damage to the applicability, safety and durability of the equipment and pipeline.

[0063] In the present invention, through rational design, the inventors have succeeded in overcoming the above-mentioned difficulties and obtaining the pipe described in the present invention.

[0064] Compared with the prior art, the pipe and its manufacturing method according to the present invention have the following advantages and beneficial effects:

[0065] In the present invention, the inventor intends to design the composition of the corrosion-resistant layer and the base layer, design the ratio of the two, assemble the corrosion-resistant layer and the base layer according to the working conditions of the pipeline equipment when used under the sodium hydroxide corrosive medium environment in the current water supply plant field, and combine the appropriate heating, rolling and coiling processes in cooperation with the pipe fabrication process to obtain a pipe with sodium hydroxide corrosion resistance, good mechanical properties and high economic efficiency, where the pipeline has a yield strength ≥ 426 MPa, a tensile strength ≥ 585 MPa, and an elongation rate ≥ 35%, and the corrosion-resistant layer has an average corrosion rate ≤ 0.05 mm / year in an environment with a temperature ≤ 40°C and a sodium hydroxide concentration ≤ 30 wt%.

[0066] In the present invention, a transition layer structure with a certain thickness is formed between the corrosion-resistant layer and the base layer through a process such as hot rolling, which realizes a perfect metallurgical bond between the corrosion-resistant layer and the base, ensuring sodium hydroxide corrosion resistance and mechanical properties while improving the compatibility and economy of the material.

[0067] The pipe manufactured by using the above composition design and process control method can solve the essential defects occurring under the sodium hydroxide corrosive medium environment of 304, 316L stainless steel or carbon steel at the site of water supply plant; the pipe can be effectively applied and used in the pipeline equipment used in the sodium hydroxide medium environment at the site of water supply plant, such as the injection pipeline in the alkaline cleaning process of the membrane pool, the flushing pipeline and the pipeline from the membrane pool to the neutralization pool, etc., and can meet the requirements for sodium hydroxide corrosion resistance and mechanical properties of the pipeline and equipment, while avoiding the secondary pollution of water quality, greatly improving the applicability, safety and durability of the pipeline equipment, and thus obtaining great economic and social benefits.

[0068] Compared with the current 304 and 316L stainless steel pipelines used in the on-site membrane pool alkaline cleaning process as injection pipelines, flushing pipelines, and membrane pool to neutralization pool pipelines, the pipes described in the present invention have high strength, easy welding and mechanical processing, and high economic efficiency. Compared with carbon steel pipes, the steel plate / steel strip of the pipes described in the present invention can be spared the pipeline corrosion-resistant coating process, and at the same time has corrosion resistance and durability that carbon steel pipes are difficult to reach, which is more energy-saving, environmentally friendly and maintenance-free. [Brief description of the drawings]

[0069] [Figure 1] FIG. 1 is a schematic diagram of the interlayer structure of a pipe according to the present invention in one embodiment. [Diagram 2] FIG. 2 is a schematic diagram of an interlayer structure of a pipe according to the present invention in another embodiment. [Diagram 3] 3 and 4 are microstructure photographs showing the bond interface between the base layer and the corrosion resistant layer of the pipe of Example 3. [Figure 4] 3 and 4 are microstructure photographs showing the bond interface between the base layer and the corrosion resistant layer of the pipe of Example 3.

[0070] Detailed Description of the Embodiments The pipe described in the present invention and its manufacturing method will be further described and illustrated below with reference to specific embodiments / examples, but these descriptions and illustrations do not constitute an inappropriate definition of the technical solutions of the present invention. EXAMPLES

[0071] Examples 1 to 7 The mass percentages of the chemical elements in the corrosion resistant layers for the pipes of Examples 1-7 are listed in Table 1.

[0072] [Table 1]

[0073] The mass percentages of the chemical elements in the base layer for the pipes of Examples 1-7 are listed in Table 2.

[0074] [Table 2]

[0075] In the present invention, the pipes of Examples 1 to 7 of the present invention are all manufactured by the following steps:

[0076] (1) A process of smelting and casting according to the chemical compositions shown in Tables 1 and 2 to prepare a slab for a corrosion-resistant layer and a slab for a base layer, respectively.

[0077] (2) Assembling the slab for the corrosion-resistant layer and the slab for the base layer to obtain a composite slab: controlling the thickness of the single layer of the corrosion-resistant layer to 0.5-20% of the total thickness of the composite slab (the ratio of the thickness of the single layer of the corrosion-resistant layer in the finished pipe / total thickness of the pipe is basically the same as the thickness ratio in this process); subjecting the prepared corrosion-resistant layer slab and base slab to pretreatment, sealing the joint surface of the slab by full-circumference welding, and then vacuuming the joint surface after sealing by welding; whether to select a preheating process is determined according to the performance requirements of the final product after assembling the solid wood. If preheating is required, the preheating temperature can be further controlled to 1150-1230°C.

[0078] (3) Hot rolling process: heating the composite slab at a temperature of 1150-1230°C, followed by rolling in multiple passes within the austenite recrystallization temperature and the non-recrystallization temperature range of the base layer slab and the corrosion-resistant layer slab, controlling the total rolling reduction rate to be 90% or more, and controlling the finish rolling temperature to be 900°C or more, for example, controlling the finish rolling temperature between 920-1050°C;

[0079] (4) Coiling process: After water cooling, the wire is coiled at a temperature of 650-700°C to obtain a hot-rolled coil;

[0080] (5) The hot-rolled coil is subjected to surface treatment by pickling or mechanical methods to remove phosphorus, and when the target product is a cold-rolled coil, it can also be further subjected to cold rolling and annealing processes, and annealing is performed after cold rolling to the target thickness, and the annealing temperature is 900-1000°C.

[0081] (6) Fabricating the pipe: forming and welding using the method of spiral welded pipe or the method of longitudinal welded pipe. The welding method can be selected from submerged arc welding, gas metal arc welding, tungsten inert gas welding, plasma arc welding, shielded metal arc welding, high frequency welding or laser welding to prepare the pipe.

[0082] It should be noted that in the present invention, the pipes of Examples 1 to 7 of the present invention are all manufactured by the processes of the above steps (1) to (6), and their chemical compositions and related process parameters all meet the control requirements of the design specifications of the present invention.

[0083] Table 3 lists specific process parameters for the pipes of Examples 1 to 7 in the steps of the above manufacturing method.

[0084] [Table 3]

[0085] It should be noted that according to the process parameters shown in Table 3 above, in Example 1, Example 3 and Example 4, after carrying out the surface treatment of the hot-rolled coil in step (5), cold rolling and annealing can be further carried out, and cold-rolled plates can be obtained correspondingly; in Examples 2, 5, 6 and 7, hot-rolled plates are obtained by step (5) without carrying out cold rolling and annealing.

[0086] Correspondingly, the plates prepared according to the various embodiments can be further used to prepare corresponding pipes in the pipe manufacturing process of step (6).

[0087] The finished pipes, i.e., the pipes of Examples 1 to 7 prepared by the above process, were sampled respectively, and the microstructures in the sample pipes of Examples 1 to 7 were further observed. After observing the microstructures of the pipes of each example, the mechanical properties and sodium hydroxide corrosion resistance of the sample pipelines of Examples 1 to 7 could be further tested. The obtained test results are listed in Table 4 below.

[0088] The relevant performance test methods are as follows: Mechanical performance test: It is carried out according to GB / T 6396-2008 "Clad steel plates -- Mechanical and technological test", and the yield strength, tensile strength and elongation rate of the sample pipes in Examples 1-7 can be obtained.

[0089] Sodium hydroxide corrosion resistance test: Each coupon of Examples 1 to 7 was placed in a pipe in an alkaline cleaning process (temperature ≦40°C, sodium hydroxide concentration ≦30wt%) of a membrane pool of a water supply plant, and after 3 months, it was removed and the surface state of the corrosion-resistant layer was observed, the reduction in corrosion weight was examined, and the presence or absence of corrosion phenomenon was judged. The annual corrosion rate of each Example was calculated. In this way, the sodium hydroxide corrosion resistance of the sample pipe of Examples 1 to 7 can be obtained.

[0090] The microstructure, mechanical properties and sodium hydroxide corrosion resistance test results of the pipes of Examples 1-7 are listed in Table 4.

[0091] [Table 4]

[0092] From Table 4 above, it can be seen that in the present invention, the pipes of Examples 1 to 7 have a yield strength of 426 to 507 MPa, a tensile strength of 585 to 650 MPa, an elongation rate of 35.8 to 40.0%, and an annual corrosion rate of sodium hydroxide in the test environment of ≦0.04 mm / year.

[0093] From the above examples, it can be seen that the present invention can obtain a pipe with good corrosion resistance and mechanical properties by combining appropriate material selection, composition design, rolling and heat treatment processes with the pipe manufacturing process. The pipe of the present invention can solve the essential shortcomings of 316L stainless steel or carbon steel occurring in the current sodium hydroxide corrosive medium environment of the water supply plant site, and can meet the ever-increasing requirements for corrosion resistance and mechanical properties of the pipe of the pipeline equipment used in the sodium hydroxide medium environment of the water supply plant site, thereby achieving great economic and social benefits.

[0094] FIG. 1 is a schematic diagram of the interlayer structure of a pipe according to the present invention in one embodiment.

[0095] As shown in FIG. 1, in this embodiment, the pipe according to the present invention can include: two corrosion-resistant layers on the inner and outer surfaces of the pipe, and a carbon steel base layer in the middle.

[0096] FIG. 2 is a schematic diagram of an interlayer structure of a pipe according to the present invention in another embodiment.

[0097] As shown in FIG. 2, in this embodiment, the pipe according to the present invention may include: a corrosion-resistant layer on the inner surface of the pipe, and a carbon steel base layer on the outer surface of the pipe.

[0098] 3 and 4 are microstructure photographs showing the bond interface between the base layer and the corrosion resistant layer of the pipe of Example 3.

[0099] As shown in Figs. 3 and 4, in Example 3, the total thickness of the pipe is 4 mm, and the thickness of the single corrosion-resistant layer is 50 µm.

[0100] It can be seen from the observation that in Example 3, the microstructure of the base layer of the pipe is ferrite + pearlite + bainite; the microstructure of the corrosion-resistant layer is austenite; correspondingly, in the present invention, the elements of the corrosion-resistant layer and the base layer diffuse at the bonding interface to form a stable transition layer, and the diffusion distance is about 50 μm.

[0101] It should be noted that the prior art part in the protection scope of the disclosure of the present invention is not limited to the examples described in the application file. All prior art contents that are not inconsistent with the technical solution of the present invention, including but not limited to prior patent documents, prior publications, prior known applications, etc., can all be incorporated into the protection scope of the present invention.

[0102] Furthermore, the manner in which the various technical features of the present invention are combined is not limited to the manner described in the claims or the manner described in the specific examples of the present invention, and all technical features described in the present invention can be freely combined or integrated in any manner as long as no contradiction occurs.

[0103] It should also be noted that the above-mentioned embodiments are merely specific examples of the present invention. Obviously, the present invention is not limited to the above-mentioned embodiments. Those skilled in the art can directly derive or easily think of similar modifications or changes thereto from the contents disclosed by the present invention. All of them belong to the protection scope of the present invention.

Claims

1. A pipe having a corrosion-resistant layer and a base layer in the thickness direction, the corrosion-resistant layer being disposed at least on the inner wall of the pipe, and the corrosion-resistant layer further comprises the following weight percentages of chemical elements in addition to Fe and unavoidable impurities: 0<C≦0.05%; Si: 0.3-0.6%; Mn: 0.5-2.0%; Ni: 8.00-14.00%; Cr: 16.00-19.00%; Mo: 2.00-3.50%; N: 0.02 to 0.20%; and Ti: 0.01-0.2%; Here, Cr, Mo, N and Ti satisfy the following inequality: Cr+2.8×Mo+16×N+2×Ti≧22.0%.

2. 2. The pipe of claim 1, wherein the corrosion resistant layer consists of the following weight percentages of chemical elements: 0<C≦0.05%; Si: 0.3-0.6%; Mn: 0.5-2.0%; Ni: 8.00-14.00%; Cr: 16.00-19.00%; Mo: 2.00-3.50%; N: 0.02-0.20%; Ti: 0.01 to 0.2%; and Residual Fe and unavoidable impurities; Here, Cr, Mo, N and Ti satisfy the following inequality: Cr+2.8×Mo+16×N+2×Ti≧22.0%.

3. 3. The pipe according to claim 1 or 2, containing the following unavoidable impurities in the corrosion-resistant layer: S≦0.030%; P≦0.045%.

4. 3. The pipe according to claim 1 or 2, wherein the base layer comprises the following weight percentages of chemical elements: C: 0.01-0.20%; Si: 0.10-0.30%; Mn: 0.50-1.50%; Al: 0.02-0.03%; Ti: 0.005-0.018%; Nb: 0.005-0.020%; N≦0.006%; and The balance is Fe and unavoidable impurities.

5. 5. The pipe of claim 4, wherein the base layer further comprises at least one of the following chemical elements: 0<B≦0.0003%; 0<Ni≦0.20%; 0<Cr≦0.20%; 0<Mo≦0.10%.

6. 5. The pipe according to claim 4, containing the following unavoidable impurities in the base layer: S≦0.010%; P≦0.015%.

7. 3. A pipe according to claim 1 or 2, wherein the thickness of a single layer of the corrosion resistant layer accounts for 0.5-20% of the total thickness of the pipe.

8. 3. The pipe of claim 1 or 2, wherein the base layer has a ferrite+pearlite or ferrite+pearlite+bainite microstructure and the corrosion resistant layer has an austenite microstructure.

9. 3. The pipe of claim 1 or 2, wherein the pipe has a yield strength of ≥ 426 MPa, a tensile strength of ≥ 585 MPa and an elongation of ≥ 35%, and the corrosion resistant layer has an average corrosion rate of ≤ 0.05 mm / year in an environment with a temperature of ≤ 40°C and a sodium hydroxide concentration of ≤ 30 wt%.

10. A method for manufacturing a pipe according to any one of claims 1 to 9, comprising the steps of: (1) providing a slab for a corrosion-resistant layer and a slab for a base layer; (2) Assembling the corrosion-resistant layer slab and the base layer slab to obtain a composite slab, in which the thickness of a single layer of the corrosion-resistant layer accounts for 0.5-20% of the total thickness of the composite slab; (3) Hot rolling process: heating the composite slab at a temperature of 1150-1230°C, followed by rolling in multiple passes with a total rolling reduction of 90% or more and a finish rolling temperature of 900°C or more; (4) Winding process: After water cooling, wind at a temperature of 650 to 700°C to obtain a hot-rolled coil; (5) subjecting the hot rolled coil to a surface treatment; and (6) A process for manufacturing pipes.

11. The method according to claim 10, wherein in the step (3), the finish rolling temperature is 920 to 1050°C.

12. The method of claim 10, wherein the method further comprises a preheating step between steps (2) and (3), and the preheating temperature is 1150-1230°C.

13. The method according to claim 10, further comprising a cold rolling and annealing step between steps (5) and (6), preferably at an annealing temperature of 900-1000°C.

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