WO2024088380A1

WO2024088380A1 - High-strength corrosion-resistant steel for photovoltaic pile foundation and manufacturing method therefor

Info

Publication number: WO2024088380A1
Application number: PCT/CN2023/127069
Authority: WO
Inventors: 宋凤明; 周庆军; 李育霖; 段争涛; 董海; 贺应广; 宋育来
Original assignee: 宝山钢铁股份有限公司
Priority date: 2022-10-28
Filing date: 2023-10-27
Publication date: 2024-05-02
Also published as: CN117947335A

Abstract

The present disclosure relates to a corrosion-resistant steel and a manufacturing method therefor, the corrosion-resistant steel comprising the following chemical components by weight percentage: C, 0.05-0.12%; Si, 0.20-0.50%; Mn, 0.5-1.2%; P≤0.018%; S≤0.006%; Al, 0.015-0.15%; Cu, 0.10-0.50%; Cr, 0.3-1.2%; Mo, 0.02-0.15%; Ni≤0.20%; N≤0.006%; and the balance being Fe and other inevitable impurity elements. In addition, the contents of Cr, Mo and Al satisfy Cr/(0.54Mo+1.93Al)≥2.0. The corrosion resistance of the corrosion-resistant steel of the present disclosure in a soil corrosion environment having a sulfate ion concentration of 300-30000 mg/kg and a chloride ion concentration of 1500-8000 mg/kg is more than 6 times that of ordinary carbon steel (less than 16.5% of the corrosion rate of ordinary carbon steel); in an atmospheric environment, compared with ordinary carbon steel, the corrosion rate thereof is ≤55%, yield strength is ≥460MPa, tensile strength is ≥540MPa, elongation A is ≥18%, and impact energy value at -40℃ is ≥160J.

Description

A kind of high strength corrosion resistant steel for photovoltaic pile foundation and its manufacturing method

Technical Field

The present disclosure relates to the field of weathering steel, and in particular to a high-strength corrosion-resistant steel that can be used for photovoltaic pile foundations and a manufacturing method thereof.

Background technique

Steel corrosion is a common and serious problem. In the soil, due to the presence of microorganisms, water, oxygen and various minerals, there is also a certain degree of corrosion on steel structures. In particular, some areas were marine environments in the Paleozoic era, so their soils contain a large amount of sulfate and chloride ions, with a content of up to thousands of mg/kg, threatening the safety of steel structures.

In order to effectively extend the service life of equipment and reduce the cost of use, corrosion-resistant steel came into being. Based on the American Corten steel, various countries have developed a series of corrosion-resistant steel products according to resources and usage requirements. In terms of strength and performance, it has developed from the early 235MPa level to the 450MPa level of high-strength weathering steel and high-corrosion-resistant steel with better corrosion resistance, and many related steel patents have also been generated.

Chinese patent publication number CN101660099B relates to "high-strength low-alloy hot-rolled ferrite bainite weathering steel and its production method". The weathering steel is designed with a higher Mn content and has a yield strength of 450MPa. However, the corrosion resistance of the weathering steel is only at the level of conventional weathering steel and does not involve soil corrosion resistance.

Chinese patent application publication number CN1986864A relates to "a high-strength low-alloy atmospheric corrosion-resistant steel and its production method", Chinese patent announcement number CN102168229B relates to "weathering steel plate and its manufacturing method", and Chinese patent application publication number CN107779740A relates to "hot-rolled steel strip with yield strength of 700MPa and its manufacturing method". The steel plates in these patents/patent applications have a higher strength of more than 450MPa, but they all use a higher Mn content in the composition design, and the cost is higher through the composite addition of strengthening elements such as Mo, Nb, V, and Ti. The weather resistance level of these steel plates is also only equivalent to that of traditional weathering steel, that is, the relative corrosion rate is ≤55%, which does not meet the requirements of soil corrosion resistance.

In order to meet the application requirements under more working conditions, weathering steel, in addition to high strength, is also developing towards high corrosion resistance and high toughness, and is required to have good machinability and lower cost.

Japanese patent application publication number JP10025550A relates to "corrosion-resistant steel", Japanese patent application publication number JP2002363704A relates to "corrosion-resistant steel with excellent toughness in base material and heat-affected zone", and Chinese patent application publication number JP10025550A relates to "corrosion-resistant steel with excellent toughness in base material and heat-affected zone". Opening number CN102127717A involves "Cr-containing weathering steel with excellent toughness and high corrosion resistance". The steel types involved in these patent applications all have better corrosion resistance and lower relative corrosion rate, but do not involve sulfate and chloride ion corrosion resistance; in addition, the content of Cr, Al, Ni and other elements in these steels is very high, which makes steelmaking difficult and the manufacturing cost high.

In a soil corrosion environment with high sulfate and chloride ion content, the sulfate ion concentration in the soil is in the range of 300-30000 mg/kg, and the chloride ion concentration is also in the range of 1500-8000 mg/kg. The corrosion of steel materials is caused by a large amount of chloride and sulfate ions in the soil, which is significantly different from the use environment of conventional weathering steel.

Chinese patent application publication number CN102268613A relates to "a hot-rolled steel plate resistant to atmospheric corrosion for railway vehicles and a method for manufacturing the same". Although the patent application focuses on sulfate and chloride ion corrosion, the steel contains 0.01-0.04% P and also needs to add appropriate amounts of Ca, Mg, Ce and Sb. Higher P is not good for low-temperature toughness and formability, while the addition of Mg and Ce increases the difficulty of production, and the addition of Sb is not good for the human body and the environment.

Chinese patent application publication number CN109023071A relates to "a steel for buried structures resistant to neutral soil corrosion and a method for manufacturing the same". In addition to containing relatively high levels of Cr (2.0-3.5%), Ni (0.2-0.4%) and Mo (0.3-0.5%), the steel also contains 0.08-0.18% Sb. Sb combined with Cu can form a _Cu2Sb protective film on the surface of the steel, thereby improving the sulfuric acid dew point corrosion resistance. However, the addition of Sb is obviously detrimental to the environment and the human body. Sb is a typical toxic and harmful heavy metal element that produces chronic toxicity and potential carcinogenicity to humans and animals. With the improvement of environmental awareness in the whole society and the tightening of environmental protection policies, the production and application of Sb-containing steel will inevitably be subject to more restrictions.

From the comparison with the existing patents, it can be found that the corrosion resistance of current weathering steel, whether it is conventional weathering steel or high corrosion resistant weathering steel, is mainly aimed at atmospheric corrosion environment, and is not suitable for soil corrosion environment with high sulfate and high chloride ion. Although there are some weathering steels for similar soil corrosion environment, Sb elements that are harmful to the environment and human health are often added.

Summary of the invention

In view of the above defects and deficiencies of the prior art, the purpose of the present disclosure is to provide a high-strength corrosion-resistant steel suitable for photovoltaic pile foundations and a manufacturing method thereof. The corrosion-resistant steel has good corrosion resistance in a soil corrosion environment with high concentrations of sulfate ions and chloride ions, and also has good atmospheric corrosion resistance and good welding performance.

To achieve the above object, the present disclosure provides a corrosion-resistant steel, which contains the following chemical components by weight percentage:

C: 0.05-0.12%, Si: 0.20-0.50%, Mn: 0.5-1.2%, P≤0.018%, S≤0.006%, Al: 0.015-0.15%, Cu: 0.10-0.50%, Cr: 0.3-1.2%, Mo: 0.02-0.15%, Ni≤0.20%, N≤0.006%, the balance is Fe and other unavoidable impurity elements,

Among them, the contents of Cr, Mo and Al satisfy Cr/(0.54Mo+1.93Al)≥2.0.

Preferably, the corrosion-resistant steel further contains one or more of Ti, Nb and V, wherein Ti: 0.01-0.06%, Nb: 0.01-0.03% and/or V: 0.01-0.04%.

Preferably, the corrosion-resistant steel further contains Sn and/or RE, wherein Sn: 0.01-0.12% and/or RE: 0.01-0.12%.

Preferably, the content of Mn is 0.6-1.0%.

Preferably, the content of Cu is 0.15-0.35%.

Preferably, the content of Cr is 0.4-0.9%.

Preferably, the corrosion-resistant steel does not contain Sb, Mg or Ce.

Preferably, the microstructure of the corrosion-resistant steel is ferrite+pearlite+bainite.

Preferably, the microstructure of the corrosion resistant steel is ferrite + pearlite and about 40-70% bainite.

Preferably, in a soil corrosion environment with a sulfate ion concentration of 300 to 30,000 mg/kg and a chloride ion concentration of 1,500 to 8,000 mg/kg, the corrosion rate of the corrosion-resistant steel relative to ordinary carbon steel is lower than 16.5%; in an atmospheric environment, the corrosion rate of the corrosion-resistant steel relative to ordinary carbon steel is ≤55%.

Preferably, the yield strength of the corrosion-resistant steel is ≥460MPa, more preferably ≥480MPa; the tensile strength is ≥540MPa, more preferably ≥580MPa; the elongation A is ≥18%, more preferably the elongation A is ≥20%; the impact energy value at -40°C is ≥160J, more preferably the impact energy value at -40°C is ≥200J.

The corrosion-resistant steel disclosed in the present invention also adopts a Cu-Cr-Mo component design, and has good corrosion resistance in a soil corrosion environment with a sulfate ion concentration of 300 to 30,000 mg/kg and a chloride ion concentration of 1,500 to 8,000 mg/kg. Preferably, the corrosion-resistant steel disclosed in the present invention selectively adds an appropriate amount of V, Ti, and Nb on the basis of a relatively low content of C-Si-Mn to produce a precipitation strengthening effect, thereby further improving the strength and achieving good low-temperature toughness. The corrosion-resistant steel disclosed in the present invention achieves a combination of corrosion resistance and high strength and high toughness at a low cost. In addition, the corrosion-resistant steel disclosed in the present invention does not contain Sb and is more environmentally friendly.

Therefore, the corrosion-resistant steel disclosed in the present invention has one or more of the following advantages:

1. The corrosion-resistant steel disclosed in the present invention has good corrosion resistance in a soil corrosion environment with a sulfate ion concentration of 300 to 30,000 mg/kg and a chloride ion concentration of 1,500 to 8,000 mg/kg (the corrosion resistance in the same corrosion environment is more than 6 times that of ordinary carbon steel, and the relative corrosion rate is lower than 16.5%), and the atmospheric corrosion resistance also reaches the level of conventional weathering steel (the corrosion rate relative to Q345B is lower than 55%), thereby meeting the corrosion resistance requirements in a variety of environments, especially the corrosion resistance requirements of photovoltaic pile foundations in soil environments with high concentrations of sulfate and chloride ions.

2. The corrosion-resistant steel disclosed in the present invention has excellent mechanical properties, yield strength ≥460MPa, tensile strength ≥540MPa, elongation A ≥18%, -40℃ impact energy value ≥160J; it also has good cold working performance and excellent cold bending performance. Good, meeting the cold bending requirements of D=2a and 180°.

3. The corrosion-resistant steel disclosed in the present invention does not contain Sb, thus avoiding harm to the environment and human health, and is an environmentally friendly product.

The present disclosure also provides a method for manufacturing the above-mentioned corrosion-resistant steel, which comprises the following steps:

1) Smelting and casting

2) Heating of ingot

The heating of the casting is carried out in a heating furnace with a reducing atmosphere. The casting temperature is above 1230°C and the holding time is 2 to 4 hours.

3) Rolling

Hot rolling is adopted, the rough rolling end temperature of the steel billet is above 1000℃, and the cumulative rough rolling reduction rate is ≥80%; the finishing rolling start temperature is ≥950℃, and the finishing rolling final temperature is 850-910℃, followed by cooling and coiling.

Preferably, in step 1), LF refining is used for smelting.

Preferably, in step 2), the soaking time is not less than 40 minutes.

Preferably, in step 3), the cooling rate is above 10°C/s and the coiling temperature is 500-560°C.

The method disclosed in the present invention has one or more of the following advantages:

1. LF refining is used in smelting, which reduces the RH link and further reduces the cost.

2. Controlled rolling and controlled cooling have a wider process window and simple process steps.

3. No heat treatment is required for rolled delivery, the production process is simple, the production cycle is short, the cost is low, and it can be implemented using existing steel rolling equipment.

4. The controlled rolling and controlled cooling process is adopted, and the high temperature finishing rolling in the range of 880-950℃ is selected in combination with the phase change temperature curve. The deformation resistance of the steel is low, which reduces the equipment load and energy consumption; at the same time, the coiling temperature is controlled at 500-560℃, which is conducive to the formation of more bainite structure and the improvement of strength. The cooling rate after rolling can be controlled at more than 10℃/s, which reduces the difficulty of cooling control and widens the process control window.

5. In addition to high strength, high toughness and good corrosion resistance, the steel produced by the method disclosed in the present invention also has good processing properties such as welding and cold bending, and has excellent elongation. It is particularly suitable for various cold forming processes in the pile foundation production process; it can also be used in other corrosive environments containing sulfate ions and chloride ions, such as seawater, gas pipelines, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a CCT phase transformation temperature curve of the steel according to the embodiment of the present disclosure.

FIG. 2 is a TTT phase transformation temperature curve of the steel according to the embodiment of the present disclosure.

FIG3 shows the microstructure of Example 1, in which the bainite content is about 61%.

FIG. 4 shows the microstructure of Example 5, in which the bainite content is about 70%.

Detailed ways

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs.

In this disclosure, plain carbon steel refers to Q345B steel, the composition of which is shown in the following table:

Table 1. Chemical composition of plain carbon steel Q345B (weight percentage/%)

In the present disclosure, the corrosion rate in a soil corrosion environment (sulfate ion concentration of 300-30000 mg/kg, chloride ion concentration of 1500-8000 mg/kg) is determined by a full immersion test. The full immersion test is carried out according to GB 10124-1988 "Method for full immersion test of uniform corrosion of metal materials in laboratory", the test solution is 10.0% _H2SO4 + _3.5 %NaCl, the test time is 24h, and the test temperature is 23±2℃.

In the present disclosure, the corrosion rate in the atmospheric environment is measured according to TB/T2375 "Cyclic Immersion Corrosion Test Method for Weathering Steel for Railway Use", such as the method described in the Examples.

In the present disclosure, the yield strength, tensile strength and elongation A are measured according to GB/T 228.1, the -40°C impact energy value is measured according to GB/T 229, and the transverse cold bending 2a and 180° cold bending are measured according to GB/T 232.

The chemical composition design principle of the corrosion-resistant steel disclosed in the present invention is as follows:

Carbon (C): C is an effective strengthening element in steel. When dissolved in the matrix, it has a solid solution strengthening effect. At the same time, C exists in the form of carbides in steel. When combined with alloying elements, it can play a role in precipitation strengthening and grain refinement. Therefore, the amount of C added should not be less than 0.05%. However, excessive C will form more carbides in the steel, which will act as a galvanic cell, promote corrosion, reduce the corrosion resistance of the steel, and be unfavorable for welding. Therefore, the C content should not be higher than 0.12%.

Silicon (Si): Si is generally added to deoxidize steel. It is also a corrosion-resistant element and has a solid solution strengthening effect. Therefore, the lower limit of Si content is controlled to 0.20%. However, a higher Si content will lead to deterioration of weldability and toughness of the heat-affected zone. Therefore, the upper limit of Si content is controlled to 0.50%.

Manganese (Mn): Mn is an important toughening element that plays a role in solid solution strengthening and can improve the strength of steel. At the same time, Mn can also expand austenite, reduce the transformation temperature of supercooled austenite, promote the transformation of medium and low temperature strengthening structure in steel, and help improve the strength of steel. However, excessive Mn content will increase hardenability, thereby deteriorating weldability and toughness of welding heat affected zone, and higher Mn also increases cost. Therefore, the present disclosure limits the Mn content range to 0.5-1.2%, preferably 0.6-1.0%.

Phosphorus (P): P is the main corrosion-resistant element in traditional atmospheric corrosion-resistant steel. It can promote the formation of a protective rust layer on the surface and effectively improve the atmospheric corrosion resistance of steel. However, P is prone to segregation at the grain boundaries, reducing the grain boundary binding energy and the toughness and plasticity of the steel. Moreover, the coexistence of P and Mn will aggravate the temper brittleness of the steel. The segregated P makes the steel plate prone to intergranular fracture and reduces the impact toughness of the steel plate. Moreover, P is not good for welding performance. Since the steel type disclosed in the present invention requires high toughness, P is controlled as an impurity element to minimize the P content in the steel. However, if the P content is controlled too low, it will increase the difficulty of steelmaking and manufacturing costs. Therefore, the P content is controlled to not exceed 0.018%.

Sulfur (S): In the present disclosure, S in steel is controlled as a harmful impurity element. S not only reduces the low temperature toughness of steel, but also promotes the anisotropy of the steel plate, which is detrimental to the cold forming performance. In addition, sulfide inclusions will significantly reduce the weather resistance of steel. Therefore, the steel grade of the present disclosure is designed to use an extremely low S content, which is controlled to be below 0.006%.

Aluminum (Al): Al is usually added to steel as a deoxidizer during the steelmaking process. Trace amounts of Al are also beneficial for grain refinement and improving the strength and toughness of steel. Adding appropriate amounts of Al increases the corrosion potential of steel, which is beneficial for inhibiting corrosion. At the same time, the formation and aggregation of nanoscale complex oxides containing Al and Si in the inner rust layer can increase the charge transfer resistance, thereby inhibiting the corrosion process. However, Al will reduce the stability of austenite, reduce the supercooling of austenite, and cause the new phase nucleus to grow rapidly, thereby reducing hardenability and increasing the critical quenching rate. In addition, Al is a ferrite-forming element. More Al will not only reduce the strength of the steel plate, but also increase the brittleness of ferrite in the steel, resulting in a decrease in the toughness of the steel. Therefore, the present disclosure controls the Al content to 0.015-0.15%.

Chromium (Cr): Cr is a precious alloy element and an effective element for improving the corrosion resistance of steel plates. Cr forms a continuous solid solution with Fe in steel, which has a solid solution strengthening effect, and forms various types of carbides with C, such as _M3C _, _M7C3 and _M23C6 , to produce a secondary strengthening effect. Cr has a significant effect on improving the passivation ability of steel, and can promote the formation of a dense passivation film or protective rust layer on the surface of steel _. Its enrichment in the rust layer can effectively improve the selective permeability of the rust layer to corrosive media. At the same time, the addition of Cr can effectively increase the self-corrosion potential of steel and improve the atmospheric corrosion resistance of steel. However, the addition of too much Cr will increase the manufacturing cost. Therefore, in order to reduce costs, the present disclosure controls the Cr content to 0.30-1.2%, preferably 0.4-0.9%.

Copper (Cu): Cu plays a major role in solid solution and precipitation strengthening in steel. At the same time, the electrochemical potential of Cu is higher than that of Fe, which can promote the formation of a dense rust layer on the steel surface and improve the corrosion resistance. It will combine with the residual S in the steel to form a Cu ₂ S protective film, which can alleviate corrosion in a high sulfate corrosion environment. However, too high Cu will not only damage the toughness of the weld heat affected zone, but also easily cause cracking during hot rolling, deteriorate the surface properties of the steel plate, and increase costs. Therefore, the present disclosure controls the Cu content to 0.10-0.50%, preferably 0.15-0.35%.

Nickel (Ni): Ni is an element that expands austenite. Ni can improve low-temperature impact toughness by refining grains and reducing stacking fault energy; at the same time, grain refinement also has a fine grain strengthening effect. In addition, Ni is also an important element for improving the corrosion resistance of steel, which can promote the stability of the rust layer and improve the hot working brittleness problem caused by Cu. However, Ni is a precious element and can be added selectively (i.e., added or not added), and the content is controlled to be below 0.2%.

Molybdenum (Mo): Mo can exist in steel in the form of solid solution, producing a solid solution strengthening effect. During quenching heat treatment, Mo promotes the formation of bainite and martensite structures, and has phase transformation strengthening and dislocation strengthening effects. At the same time, Mo can increase the solubility of Nb, V and Ti, and promote precipitation strengthening. In low-alloy high-strength steel, the strength of steel increases significantly with the increase of Mo content. The addition of Mo can improve the corrosion product film structure and improve the local corrosion resistance of steel such as pitting corrosion and crevice corrosion. In the present disclosure, the synergistic effect of Cr and Mo is utilized, and supplemented with appropriate Al, which significantly improves the corrosion resistance of steel in a soil corrosion environment with a sulfate ion concentration of 300 to 30,000 mg/kg and a chloride ion concentration of 1,500 to 8,000 mg/kg. However, too high Mo is not good for welding performance and has a high cost. Therefore, the present disclosure controls the Mo content to 0.02 to 0.15%, and requires that the contents of Cr, Mo and Al satisfy Cr/(0.54Mo+1.93Al)≥2.0.

Nitrogen (N): N can form nitrides with Al and Ti in steel. Fine nitride precipitates have the effect of pinning grain boundaries and thus can refine austenite grains. Higher N is easy to form AlN when combined with Al in steel, which significantly increases the number of nitrides in the steel. When AlN exists independently in steel as a non-metallic inclusion, it will destroy the continuity of the steel matrix. Especially when the Al content is high, the amount of AlN formed is large and distributed in an aggregated manner, the degree of harm is even greater, and at the same time, oxides with poor plasticity are formed. Moreover, higher N is easy to enrich in defects and deteriorate low-temperature impact toughness. Therefore, the present disclosure requires that the N content be controlled below 0.0060%.

In addition to the above elements, in order to further improve the performance, the steel disclosed in the present invention may further selectively add one or more of Nb, V, Ti, Sn and RE.

Titanium (Ti): Ti is a strong ferrite-forming element and carbonitride-forming element, and it is easy to form compounds with C, N, O, S, etc. Ti exists in steel mainly in the form of TiC or Ti(C, N). The purpose of adding Ti in the present disclosure is mainly to use TiN to inhibit the growth of austenite grains and refine the structure; at the same time, it produces precipitation strengthening during the cooling process. In addition, Ti has the effect of preventing the recrystallization of deformed austenite and promoting the formation of granular bainite. The precipitated Ti carbonitride particles can prevent the grain coarsening of the heat-affected zone of welding and improve the welding performance. However, Ti content When the amount is too high, the titanium nitride particles tend to grow and agglomerate at high temperatures, which will damage the plasticity and toughness of the steel. Therefore, when adding Ti, the present disclosure controls the Ti content to be 0.01-0.06%.

Niobium (Nb): Nb is a strong nitrogen carbide-forming element. During the cooling process after rolling, it can combine with carbon and nitrogen in steel to form intermediate phases such as NbC, Nb (CN) and NbN. The fine carbide particles formed can refine the structure, produce fine grain strengthening and precipitation strengthening, and significantly improve the strength of the steel plate. At the same time, the refinement of the structure is conducive to the improvement of the toughness of the steel plate. In addition, Nb can inhibit the expansion of the austenite interface, increase the recrystallization temperature of the steel, and realize non-recrystallization zone rolling at a higher temperature. Therefore, adding an appropriate amount of Nb to the steel is conducive to improving the strength. However, when the Nb content is high, coarse carbonitride particles will be formed at the grain boundary, which will deteriorate the impact toughness. At the same time, Nb is a precious alloying element. Therefore, when adding Nb, the Nb content is controlled to 0.01-0.03%.

Vanadium (V): V is a strong carbonitride-forming element, which can precipitate during phase transformation, has solid solution strengthening and carbonitride precipitation strengthening effects in steel, and increases tempering stability, thereby improving strength. Therefore, when adding V, the V content is controlled to 0.01-0.04%.

Tin (Sn): Sn has a good corrosion inhibition effect in steel. At the same time, Sn ions can dissolve in the anode to inhibit the anode reaction and reduce the formation of β-FeOOH that is unfavorable to corrosion resistance. Therefore, when adding Sn, the Sn content is controlled to 0.01-0.12%.

Rare earth (RE): RE compounds, RE/Fe intermetallic compounds and solid-solution rare earth formed by RE in steel are hydrolyzed in the thin corrosion film and precipitated at the cathode with a higher pH value, thereby playing a corrosion inhibition role. Therefore, when adding RE, the RE content is controlled to 0.01-0.12%.

Through the above-mentioned component design, the corrosion-resistant steel disclosed in the present invention has good corrosion resistance in a soil corrosion environment with a sulfate ion concentration of 300 to 30,000 mg/kg and a chloride ion concentration of 1,500 to 8,000 mg/kg, and at the same time, the atmospheric corrosion resistance is comparable to that of conventional weathering steel, especially meeting the corrosion requirements of photovoltaic pile foundations in soil environments with high concentrations of sulfate and chloride ions, and can also be used in equipment such as seawater and gas pipelines that are corroded by chloride ions and sulfate ions, as well as conventional steel structure production that requires atmospheric corrosion resistance. In addition, the corrosion-resistant steel disclosed in the present invention also has good forming performance and low-temperature impact toughness, meets the use and processing requirements of photovoltaic pile foundation steel, and is also suitable for seawater environments with high chloride ion content and gas pipelines with high sulfate ions.

The corrosion-resistant steel disclosed in the present invention adopts a Cu-Cr-Mo component system design, and achieves a significant improvement in corrosion resistance in a soil corrosion environment with high concentrations of sulfate and chloride ions through the synergistic effect of multiple corrosion-resistant elements.

Cu is an effective element to improve sulfate ion corrosion. Cr is a commonly used corrosion resistant element. In addition to solid solution strengthening and improving hardenability, the addition of Cr also increases the corrosion potential. However, adding only Cu and Cr cannot achieve the improvement of corrosion resistance in soil corrosion environments with high concentrations of sulfate ions and chloride ions. Studies have shown that steel Adding about 1% Cr can increase the self-corrosion potential by about 40 mA. Corrosion evaluation shows that when the potential difference is less than 70 mA, it will not cause obvious galvanic corrosion. Therefore, a corrosion potential difference of 40 mA will not significantly change the corrosion current, change the corrosion tendency, and has little effect on improving corrosion resistance. Cu is a must-add element in acid-resistant steel at present, but at the same time, about 0.1% Sb must be added to obtain the expected corrosion resistance. The present disclosure abandons the addition of Sb from the perspective of the environment and human health. However, relying solely on Cr and Cu cannot obtain excellent corrosion resistance in soil environments with high concentrations of sulfate ions and chloride ions.

The key to the good corrosion resistance of the corrosion-resistant steel disclosed in the present invention in the soil environment with high concentration of sulfate and chloride ions is to utilize the synergistic effect of Cr and Mo. Mo is usually used in high-strength steel to improve hardenability and tempering stability, or Mo is used to inhibit pitting corrosion of high-Cr corrosion-resistant steel, thereby improving the corrosion resistance of steel in seawater, but its content is usually above 0.2%, or even as high as 0.5%, which significantly increases the cost. The inventors have found that in an environment with high sulfate and chloride ions, no more than 0.15% of Mo is combined with Cr, which can significantly improve the corrosion resistance of steel. The inventors also found that the Mo content is not linearly related to the improvement effect of corrosion resistance. It is speculated that this special synergistic effect may be related to the fact that Cr and Mo belong to the same group in the periodic table. Both belong to the sixth sub-group elements and are adjacent, so they have similar chemical properties and electronic characteristics and are easy to cooperate. The synergistic effect of the two simultaneously improves the self-corrosion potential and charge transfer resistance of steel in this corrosive environment, and reduces its corrosion current density. In addition, the enrichment of Mo in the rust layer effectively hinders further corrosion of the substrate, thereby significantly improving its corrosion resistance.

In addition, the corrosion-resistant steel disclosed in the present invention also adds 0.015-0.15% Al. Al itself is relatively active and easily reacts with oxygen in the air. Al has a passivation effect in the natural environment and can form a layer of Al ₂ O ₃ film on the surface of the steel, thereby achieving corrosion resistance. Therefore, a trace amount of Al is usually added to the steel as a deoxidizing element.

The inventors have found that Cr and Al form intermetallic compounds Fe ₂ CrAl and Cr ₈ Al ₅ in steel. In a high concentration sulfate and chloride ion environment, these two compounds gather on the surface of the steel, thereby improving the corrosion resistance of the steel.

The corrosion-resistant steel disclosed in the present invention utilizes Cu and Cr to improve the matrix potential. Cu combines with S in the steel to improve the corrosion resistance in a sulfate environment. At the same time, Cr is combined with Mo and Al in a certain ratio, which is beneficial to good corrosion resistance in a soil environment with high concentrations of sulfate ions and chloride ions. The content of the three is required to satisfy Cr/(0.54Mo+1.93Al)≥2.0. In addition to Al, the other three alloys also improve the strength at the same time. Among them, Cu, Mo, and Cr all have a solid solution strengthening effect. Cu can also be dispersed and precipitated in the steel matrix to improve the strength; the addition of Mo and Cr also improves the hardenability and tempering resistance of the steel, thereby improving the strength.

It is through the combination of the above-mentioned elements that the steel plate achieves the comprehensive performance of high strength and acidic medium corrosion resistance, so that the corrosion resistance in the above-mentioned environment is more than 6 times that of ordinary carbon steel (relative corrosion rate is less than 16.5%). It has atmospheric corrosion resistance comparable to conventional weathering steel, with a corrosion rate of ≤55% relative to ordinary carbon steel, thus meeting corrosion resistance requirements in a variety of environments. The new corrosion-resistant component system disclosed in the present invention avoids the toxic pollution of Sb in conventional sulfate ion corrosion-resistant steel to the environment and human body, while greatly improving corrosion resistance, and is an environmentally friendly product.

The corrosion-resistant steel disclosed in the present invention utilizes the solid solution strengthening and phase transformation strengthening of C and Mn and the precipitation strengthening and fine grain strengthening effects of V, Nb and Ti to obtain a yield strength of more than 460 MPa, and at the same time has good low-temperature impact toughness (the low-temperature impact energy value at -40°C exceeds 160J), an elongation of more than 18%, achieving a match between high corrosion resistance and high toughness, and has good forming properties, meeting the use requirements of pile foundation steel.

The matrix structure of the corrosion-resistant steel of the present application is mainly ferrite + pearlite + bainite. Bainite is stronger than ferrite, so the strength is increased by forming an appropriate amount of bainite. From the actual metallographic perspective, the content of bainite is about 40-70%.

The method for manufacturing corrosion-resistant steel disclosed in the present invention comprises the following steps:

1) Smelting and casting

2) Heating of ingot

3) Rolling

Hot rolling is adopted, the rough rolling end temperature of the steel billet is above 1000℃, the accumulative rough rolling reduction rate is ≥80%; the finishing rolling start temperature is ≥950℃, the final rolling temperature is 850～910℃, and then cooling and coiling are carried out.

In a preferred embodiment, in step 1), LF refining is used for smelting.

In a preferred embodiment, in step 2), the soaking time is not less than 40 minutes.

In a preferred embodiment, in step 3), the cooling rate is above 10°C/s, and the coiling temperature is 500-560°C.

In the method for producing corrosion-resistant steel of the present disclosure:

The heating of the ingot requires a reducing atmosphere in the heating furnace and the ingot outlet temperature should be controlled above 1230°C.

Considering that a small amount of Ti is added to the steel disclosed in the present invention, in order to ensure that the carbonitride of Ti is fully dissolved, the heating temperature is selected to be above 1230°C, the holding time is 2 to 4 hours, and the soaking holding time is preferably not less than 40 minutes. In addition, the ingot can be hot-charged into the furnace after the casting is completed, that is, after confirming that there is no quality problem on the surface of the ingot, it is directly transported from the casting area to the heating furnace through the roller for heating and heat preservation, thereby reducing energy consumption.

The rolling adopts hot continuous rolling. The method disclosed in the present invention requires that the rough rolling end temperature be controlled to be not less than 1000°C, and the rough rolling cumulative reduction rate be controlled to be ≥80%, and the finishing temperature of the finishing rolling be 850-910°C.

The Cr and a small amount of Al added to the corrosion-resistant steel disclosed in the present invention are both ferrite-forming elements, which reduce the stability of low austenite and reduce the supercooling of austenite, thereby improving the stability of ferrite formation. As shown in Figure 1, the temperature at which ferrite begins to form under continuous cooling conditions is about 852°C. In order to avoid a sudden change in rolling force caused by rolling in the two-phase region, all finishing rolling is carried out in the austenite region. If the finishing temperature is too high, the starting temperature of finishing rolling must be increased accordingly, resulting in an excessively high rough rolling temperature, which correspondingly increases the heating temperature of the ingot and increases energy consumption. Therefore, the method disclosed in the present invention limits the finishing temperature of finishing rolling to 850-910°C, and the starting temperature of finishing rolling is not lower than 950°C. For this reason, it is required to control the ending temperature of rough rolling of the steel billet to be above 1000°C.

The method disclosed in the present invention requires that the coiling temperature be controlled at 500-560°C. From the phase diagram, the steel type disclosed in the present invention forms ferrite structure in a wide range; after the temperature is reduced to 607°C, bainite phase transformation begins to occur to form bainite structure. Figure 3 is the microstructure morphology of Example 1, in which the bainite content is about 61%. Figure 4 is the microstructure of Example 5, in which the bainite content is about 70%. Pearlite is a high C component and is easy to form a galvanic cell in the matrix, which promotes the occurrence of corrosion. In order to improve the corrosion resistance of steel, the formation of pearlite in the matrix should be minimized. From the continuous cooling curve (CCT), a cooling rate of more than 10°C/s can reduce the formation of pearlite during the cooling process. In order to obtain better strength and toughness, it is required to water cool immediately after rolling to refine the structure as much as possible, and it is required to control the cooling rate after rolling to more than 10°C/s.

In order to obtain high yield strength, more bainite strengthening phases are required to be formed in the matrix. The steel type disclosed in the present invention is designed with a Cr-Mo corrosion-resistant component system. On the one hand, better corrosion resistance is obtained, and at the same time, the austenite region is reduced by Cr and Mo, wherein Cr reduces the diffusion rate of C in austenite, reduces the critical cooling rate, and improves the hardenability of the steel plate. The addition of Cr can separate the pearlite and bainite transformation curves, so that the proeutectoid ferrite and pearlite phase transformation curves shift to the right, and reduce Bs more, while reducing the Ms point less, which is beneficial to reduce the tendency of welding cracks, while refining the structure and reducing the ductile-brittle transition temperature. Mo inhibits the formation of polygonal ferrite and pearlite in steel, and promotes the formation of bainite with a large number of dislocations distributed in the crystal within a larger cooling rate range, thereby improving the strength.

From the TTT temperature curve of FIG2 , the pearlite transformation end temperature of the steel disclosed in the present invention is about 600°C, so it is required to control the coiling temperature not to exceed 600°C. About 525°C is the temperature at which bainite forms fastest. In order to form as much bainite as possible in the matrix to obtain higher strength, it is most preferred to coil at this temperature. The steel of the present invention does not require post-rolling heat treatment, shortening the production cycle and reducing production costs.

Example

The corrosion-resistant steel and the method for manufacturing the same disclosed herein are further described below in conjunction with the embodiments and drawings.

Table 2 shows the composition of the steel of Examples 1-12 and Comparative Examples 1-4. Table 3 shows the production process parameters of the steel of Examples 1-12 and Comparative Examples 1-4. Table 4 shows the performance parameters of the steel of Examples 1-12 and Comparative Examples 1-4.

The steels of Comparative Examples 1 and 2 are highly corrosion-resistant and weathering steels, wherein the steel of Comparative Example 1 is disclosed in Chinese Patent Application Publication No. CN102127717A, and the steel of Comparative Example 2 is disclosed in Chinese Patent Application Publication No. CN102268613A. The steels of Comparative Examples 3 and 4 are 450MPa grade conventional weathering steels, wherein the steel of Comparative Example 3 is disclosed in Chinese Patent Application Publication No. CN109023071A.

As can be seen from Table 2, the chemical composition of the steels of Examples 1-12 is significantly different from that of the steels of Comparative Examples 1-4. The steels of Examples 1-12 adopt a Cu-Cr-Mo component system. Comparative Example 1 is designed with a high Cr-Ni component, in which the Cr content is 3.649% and the Ni content is 0.284%, which is much higher than the steels of Examples 1-12. Comparative Example 2 not only contains Sb, but also contains a relatively high P and 3.697% Cr, and requires the addition of Mg and Ce at the same time. The low-temperature toughness and forming properties of Comparative Example 2 are not good. The higher content of Cr increases the manufacturing cost and the addition of Mg and Ce increases the difficulty of production. In particular, Sb is a typical toxic and harmful heavy metal element, which produces chronic toxicity and potential carcinogenicity to humans and animals. In addition to Sb, the content of Cr and Mo in Comparative Example 3 is also very high.

The corrosion rate in the atmospheric environment is tested according to TB/T2375 "Test method for periodic immersion corrosion of weathering steel for railway use"; the simulated soil environment is a high sulfate ion + chloride ion corrosion environment (sulfate ion concentration is 300-30000 mg/kg, chloride ion concentration is 1500-8000 mg/kg), the full immersion test is carried out according to GB 10124-1988 "Full immersion test method for uniform corrosion of metal materials in laboratory", the test solution is 10.0% _H2SO4 +3.5%NaCl, the test time is 24h, _and the test temperature is 23±2℃.

The results show that in the simulated soil environment of high sulfate ion + chloride ion, the corrosion resistance of Examples 1-12 is more than 6 times that of ordinary carbon steel (Q345B), and the corrosion rate relative to ordinary carbon steel is lower than 16.5%; the atmospheric corrosion resistance also reaches the level of conventional weathering steel, and has good corrosion resistance in soil corrosion environments (simulated soil environments) with sulfate ion concentrations of 300-30000 mg/kg and chloride ion concentrations of 1500-8000 mg/kg and in atmospheric environments, thereby meeting the corrosion resistance requirements in various environments. In contrast, although Comparative Examples 1 and 2 also have corrosion resistance that exceeds that of ordinary weathering steel in atmospheric environments, the relative corrosion rate in the simulated soil environment of high sulfate ion + chloride ion (sulfate ion concentrations of 300-30000 mg/kg and chloride ion concentrations of 1500-8000 mg/kg) is much higher than that of the steel of Examples 1-12, and the low-temperature impact toughness of Comparative Examples 1 and 2 is poor; Comparative Examples 3 and 4 are similar.

In addition, it can be seen from Table 4 that the mechanical properties of the steels of Examples 1-12 are also significantly better than those of Comparative Examples 1-4. The yield strength of the steels of Examples 1-12 is ≥460MPa, reaching 468-558MPa; the tensile strength exceeds 540MPa, reaching 584-682MPa, the elongation is above 18%, and the -40°C impact energy value exceeds 160J (10*10*55mm standard size specimen).

In addition, the process of Examples 1-12 adopts a hot rolling process, and the finishing rolling stage is carried out in the full austenite stage. During the post-rolling layer cooling process, it is only required to control the cooling rate above 10°C/s. The process window is wider, which reduces the production difficulty and facilitates on-site production. However, Comparative Example 1 requires the post-rolling cooling rate to be controlled at 5 to 20°C/s, and the cooling rate range is clearly limited, which obviously increases the difficulty of controlling water cooling; Comparative Example 2 requires final rolling at 880 to 950°C, air cooling for 1 to 35 seconds, and then cooling to 550 to 690°C at a cooling rate of more than 10°C/s for coiling. The cooling process of air cooling and water cooling after rolling obviously increases the difficulty of production. The degree of cooling, especially air cooling, leads to longer production time and affects the production rhythm.

All publications, patent applications, patents, and other references mentioned in this disclosure are incorporated by reference in their entirety.

Although the present disclosure has been illustrated and described with reference to certain preferred embodiments of the present disclosure, it should be understood by those skilled in the art that the above contents are further detailed descriptions of the present disclosure in combination with specific embodiments, and it cannot be determined that the specific implementation of the present disclosure is limited to these descriptions. Those skilled in the art may make various changes in form and details, including making several simple deductions or substitutions, without departing from the spirit and scope of the present disclosure.

Claims

A corrosion-resistant steel comprising the following chemical components by weight percentage:

C: 0.05-0.12%, Si: 0.20-0.50%, Mn: 0.5-1.2%, P≤0.018%, S≤0.006%, Al: 0.015-0.15%, Cu: 0.10-0.50%, Cr: 0.3-1.2%, Mo: 0.02-0.15%, Ni≤0.20%, N≤0.006%, the balance is Fe and other unavoidable impurity elements,

Among them, the contents of Cr, Mo and Al satisfy Cr/(0.54Mo+1.93Al)≥2.0.
The corrosion-resistant steel according to claim 1, characterized in that the corrosion-resistant steel further contains one or more of Ti, Nb and V, wherein Ti: 0.01-0.06%, Nb: 0.01-0.03% and/or V: 0.01-0.04%.
The corrosion-resistant steel according to claim 1 or 2, characterized in that the corrosion-resistant steel further contains Sn and/or RE, wherein Sn: 0.01-0.12% and/or RE: 0.01-0.12%.
The corrosion-resistant steel according to any one of claims 1 to 3, characterized in that the corrosion-resistant steel satisfies one or more of the following: a Mn content of 0.6 to 1.0%, a Cu content of 0.15 to 0.35% and a Cr content of 0.4 to 0.9%.
The corrosion-resistant steel according to any one of claims 1 to 4, wherein the corrosion-resistant steel does not contain Sb, Mg or Ce.
The corrosion-resistant steel according to any one of claims 1 to 5, characterized in that the microstructure of the corrosion-resistant steel is ferrite+pearlite+bainite.
The corrosion-resistant steel according to any one of claims 1 to 6, characterized in that, in a soil corrosion environment with a sulfate ion concentration of 300 to 30,000 mg/kg and a chloride ion concentration of 1,500 to 8,000 mg/kg, the corrosion rate of the corrosion-resistant steel relative to ordinary carbon steel is less than 16.5%; in an atmospheric environment, the corrosion rate of the corrosion-resistant steel relative to ordinary carbon steel is ≤55%.
The corrosion-resistant steel according to any one of claims 1 to 7, characterized in that the yield strength of the corrosion-resistant steel is ≥460MPa, the tensile strength is ≥540MPa, the elongation A is ≥18%, and the -40°C impact energy value is ≥160J.
A method for manufacturing the corrosion-resistant steel according to any one of claims 1 to 8, characterized in that it comprises the following steps:

1) Smelting and casting

Smelting and casting into a billet according to any one of claims 1 to 8;

2) Heating of ingot

The heating of the casting is carried out in a heating furnace with a reducing atmosphere. The casting temperature is above 1230℃ and the holding time is 2 to 4 hours;

3) Rolling

Hot rolling is adopted, the rough rolling end temperature of the steel billet is above 1000℃, and the cumulative rough rolling reduction rate is ≥80%; the finishing rolling start temperature is ≥950℃, and the finishing rolling final temperature is 850-910℃, followed by cooling and coiling.
The method according to claim 9, characterized in that, in step 1), the smelting adopts LF refining.
The method according to claim 9 or 10, characterized in that in step 2), the soaking time is not less than 40 minutes.
The method according to any one of claims 9 to 11, characterized in that in step 3), the cooling rate is above 10°C/s and the coiling temperature is 500-560°C.