High-strength steel with anti-fatigue and anti-cracking properties and method for manufacturing same

Abstract

The present application relates to a kind of high-strength steel of anti-fatigue and crack arrest and its preparation method, it relates to metal material technical field, it is by the following chemical components by mass percentage composition: C:0.029-0.079%, Si:0.09-0.29%, Mn:1.41-1.69%, Nb:0.019-0.029%, Zr:0.0041-0.0199%, Mg:0.0014-0.0199%, the rest is Fe and inevitable impurity;Zr element and Mg element mass percentage also simultaneously satisfy formula: 0.0041%<Zr+Mg<0.0199%, and Zr / Mg=1-3.The high-strength steel plate of the present application adopts low-carbon, low-silicon, middle-manganese inexpensive chemical composition design, does not contain Cr, Ni, Cu and other valuable metal elements, substantially reduces material cost;The present application does not use traditional Al deoxidation technology, instead is Si-Mn initial deoxidation, Zr-Mg final composite deoxidation, forms small, dispersed and uniform composite oxysulfide, substantially improves plasticity and toughness, significantly improves anti-fatigue performance, is suitable for the high-strength steel of anti-fatigue performance requirement high offshore wind power equipment equipment.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metal materials, in particular to a high-strength steel with fatigue resistance and crack arrest and a preparation method thereof. BACKGROUND

[0002] The appearance of fatigue failure began in the early 19th century. After the industrial revolution, with the development of steam locomotives and other means of transport, and the wide application of mechanical equipment, the destruction of moving parts often occurs. The destruction often occurs at the cross-section mutation of the component, and the nominal stress is not high, which is lower than the tensile strength and yield strength of the material. With the gradual deepening of human understanding of fatigue, many kinds of classification of fatigue research have been produced.

[0003] Material fatigue is to study the fatigue properties of the material itself, to conduct fatigue test on standard samples, and then to analyze the failure mechanism, chemical composition and microstructure, to obtain the factors affecting the fatigue properties and to improve the metallurgical process. Material fatigue is an important basis for evaluating materials in fatigue design. According to the failure cycles, it can be divided into high-cycle fatigue and low-cycle fatigue. The failure caused by the cyclic stress lower than the yield strength of the material after 107 cycles is called high-cycle fatigue; the failure caused by the cyclic stress close to or higher than the yield strength of the material after 105 cycles is called low-cycle fatigue. The main difference between high-cycle fatigue and low-cycle fatigue is the degree of plastic strain. 5 5

[0004] Fatigue fracture of offshore engineering is one of the main damage forms of marine structures, which has attracted the attention of ship classification societies and related research institutions. There are many factors that affect the fatigue life of components. In general, they can be divided into internal factors and external factors. Internal factors refer to the fatigue properties of the material itself. There are many external factors, the main influencing factors are size effect, average stress, surface factor, residual stress, etc. The fatigue properties of the material itself, as one of the main parameters affecting the fatigue life of the component, are relatively stable, which has a great relationship with the metallurgical process and the structure of the material at the micro level. The external factors, which are also parameters affecting the life of the component, have a great impact on the result of the fatigue life, and a small change can have a great impact on the fatigue life.

[0005] Under the background of carbon neutrality, the state vigorously develops the deep-sea wind power industry, and the steel for deep-sea wind power is an important guarantee for the rapid development of the wind power industry. China's deep-sea wind power is mainly concentrated in the vast sea areas of the Yellow Sea, the East China Sea and the South China Sea. Under the above service environment, the steel for deep-sea wind power, especially the steel for wind power tower and floating body components, not only needs to have high strength, but also needs to have excellent welding performance and fatigue performance, otherwise accidents will cause significant economic losses.

[0006] ​​In recent years, with the development of the "Arctic Route", more and more polar ships are put into the development and expansion of the Arctic Route, and the demand for polar ships with icebreaking capacity is increasing. Generally, there are two ways to break ice, one is continuous icebreaking, and the other is collision icebreaking. No matter which way is used to break ice, the icebreaking area of the ship will be continuously impacted by ice, causing damage to the ship. Therefore, the development of polar ships puts higher requirements on the performance of steel plates for shipbuilding. First of all, as a material for long-term service in the ice area, it should have good low-temperature toughness, and secondly, the steel plate should have high strength and high fatigue strength to withstand the continuous impact of ice.

[0007] From the above analysis, it can be seen that whether it is offshore wind power or polar icebreaking, the high strength steel for offshore engineering has higher and higher requirements on strength and toughness, weldability and fatigue resistance. The following will briefly analyze the currently disclosed related patent technologies.

[0008] The patent document CN112522631A discloses a kind of wind power steel and its preparation method, wherein the content of nickel, chromium, vanadium and titanium added is relatively high, and molybdenum element is also added, especially nickel and chromium resources are relatively scarce, which should be reduced as much as possible, so the preparation method cost of the patent is relatively high.

[0009] The patent document CN112342459A discloses a kind of low-temperature-resistant wind power flange steel and its rolling method, the yield strength of the steel plate in the specific embodiment of the patent is ≥360MPa, but below 384MPa, the impact energy is less than 75J, so the yield strength and impact energy of the low-temperature-resistant wind power flange steel prepared by the patent are lower than the development needs of modern wind power steel, the strength and toughness are poor, and the low-temperature impact resistance is poor.

[0010] The patent document CN107267863A discloses a kind of low-cost low-carbon equivalent wind power tower cylinder steel plate and its production method, the thickness specification of the steel plate of the patent is 18-40mm, the yield strength is ≥350MPa, the tensile strength is 470-630MPa, and the longitudinal impact energy at-40℃ is ≥35J; It can be seen from the embodiment that the yield strength of the wind power steel plate prepared by the patent is not higher than 408MPa, and the V-type longitudinal impact energy at-40℃ is not higher than 55J, so the yield strength and core impact energy of the steel plate at-40℃ are difficult to meet the development needs of modern wind power steel.

[0011] Comparative patent document 201910712227.1 discloses a yield strength 345 MPa grade high fatigue structural steel, the chemical composition of which is: C 0.13%-0.16%, Mn 1.30%-1.60%, Nb 0.020%-0.050%, Alt 0.020%-0.030%, Ti≤0.010%, Si≤0.12%, P≤0.010%, S≤0.005%, the balance being iron and unavoidable impurities. By adopting large reduction + controlled cooling process, the obtained steel plate has good comprehensive mechanical properties and better surface quality. However, the impact toughness of the steel plate is only evaluated at-20℃, which is far from meeting the use requirements.

[0012] Comparative patent document 201180044623.3 discloses a high-strength hot-rolled steel plate with excellent fatigue resistance, the chemical composition of which is: C 0.05-0.15%, Si 0.2-1.2%, Mn 1.0-2.0%, P 0.03% or less, S 0.0030% or less, Al 0.005-0.10%, N 0.006% or less, the remaining elements also containing Ti 0.03-0.13%, Nb 0.02-0.10%, V 0.02-0.15 one or more than two, by adopting controlled rolling and controlled cooling process, the obtained steel plate has a strength of 780 MPa or more, and a fatigue strength of 580 MPa or more under 2 million cycles, but the low temperature performance of the steel plate is not evaluated.

[0013] Comparative patent document 201810007814.6 discloses a high crack and fatigue strength thick steel plate, the chemical composition of which is: C 0.05-0.07%, Si 0.10-0.20%, Mn 1.40-1.60%, Nb 0.04-0.06%, Ti 0.01-0.02%, Cu 0.30-0.35%, Cr 0.27-0.31%, Ni 0.4-0.5%, Al 0.01-0.04%, Mo 0.06-0.11%, P≤0.020%, S≤0.010%, the balance being iron and impurities. The yield strength of the steel of the invention is not less than 500 MPa, the impact energy at-60℃ is greater than 250 J, and the fatigue strength under 2 million cycles is greater than 160 J. The fatigue strength is low, which affects the service performance of the steel plate.

[0014] The patent document 201810783890.6 discloses a high fatigue bridge steel plate, which has a chemical composition of C 0.05-0.08%, Si 0.12-0.18%, Mn 1.4-1.6%, Nb 0.045-0.058%, Ti 0.01-0.02%, Cu 0.30-0.35%, Cr 0.22-0.30%, Ni 0.45-0.55%, Al 0.02-0.04%, Mo 0.05-0.12%, P≤0.009%, S≤0.005%, and the rest is Fe and other inevitable impurities; the fatigue strength of the steel plate is not less than 170 MPa after 100 million times of fatigue, but the chemical composition is relatively high, the production cost is high, and the fatigue strength is also relatively low, which is not conducive to the service performance of the steel plate.

[0015] In summary, the production of the anti-fatigue and crack-arrest high-strength steel currently mainly has the following problems: the content of valuable alloy elements (Ni, Cu and Mo) is relatively high, the production cost is high, the impact toughness of the steel plate is insufficient, the fatigue performance of the steel plate is relatively low, and the service performance of the steel plate is affected. Therefore, it is necessary to develop a wind power equipment steel with high strength, good weldability, anti-fatigue performance and crack-arrest performance, so as to meet the market demand of offshore wind power steel, especially the demand of deep-sea wind power equipment, and be conducive to the sustainable development of the national economy. SUMMARY

[0016] The technical problem to be solved by the present application is to provide an anti-fatigue and crack-arrest high-strength steel and a preparation method thereof. The purpose is to solve the problems of insufficient anti-fatigue and crack-arrest performance of traditional high-strength steel and high alloy cost.

[0017] The present application solves the above technical problems, and the first aspect is to provide an anti-fatigue and crack-arrest high-strength steel, which is composed of the following chemical components in terms of mass percentage: C: 0.029-0.079%, Si: 0.09-0.29%, Mn: 1.41-1.69%, Nb: 0.019-0.029%, Zr: 0.0041-0.0199%, Mg: 0.0014-0.0199%, and the rest is Fe and inevitable impurities; the mass percentage of Zr and Mg elements also satisfies the formula: 0.0041%<Zr+Mg<0.0199%, and Zr / Mg=1-3.

[0018] In the anti-fatigue and crack-arrest high-strength steel of the present application, each element plays the following roles:

[0019] C element: carbon is the main strengthening element in steel, which is the main element to improve the hardenability of steel, and its solid solution strengthening and precipitation strengthening can significantly affect the mechanical properties of steel; when the content is low, the generation amount of carbide and other substances will be reduced, which will affect the effect of refining grains during rolling; when the content is high, the content of steel permeable body increases, which is not conducive to the low temperature toughness, local corrosion resistance and welding performance of the steel plate; therefore, considering the cost, performance and other factors, the mass content of C in the application is controlled to be 0.029-0.079%;

[0020] Si element: silicon is one of the main elements for steelmaking deoxidization, and has strong solid solution ability in steel, which can improve the elastic limit, yield strength and fatigue strength of steel; however, when the content of silicon is too high, it has adverse effects on the low temperature toughness and surface quality of the steel; the mass content of Si in the application is controlled to be 0.09-0.29%;

[0021] Mn element: manganese can delay the transformation of ferrite and pearlite in steel, significantly increase the hardenability of steel, reduce the brittle transition temperature of steel, and improve the impact toughness; however, when the content of Mn is too high, it is easy to form central segregation in steel, which has adverse effects on the plasticity, toughness and fatigue performance of the steel; considering the above factors, the mass content of Mn in the application is controlled to be 1.41-1.69%;

[0022] Nb element: niobium is an important grain refining element in steel, and the undissolved Nb carbon and nitride particles distributed on the austenite grain boundary can hinder the growth of austenite grains during heating; at the same time, it can effectively delay the recrystallization of deformed austenite and prevent the growth of austenite grains, thereby refining ferrite grains; therefore, niobium can improve the impact toughness of steel and reduce the brittle transition temperature; the mass content of Nb in the application is controlled to be 0.019%-0.029%;

[0023] Zr element: zirconium is a strong carbide forming element, a strong deoxidizing element and a complex oxysulfide forming element, and a small amount of zirconium has the effects of degassing, purification and grain refinement, which is beneficial to improve the low temperature performance of stainless steel and improve the stamping performance; when dissolved in austenite, it can significantly improve the hardenability of steel; therefore, the mass content of zirconium in the application is 0.0041-0.0199%;

[0024] Mg: magnesium is a strong deoxidizing element and a complex oxysulfide forming element, and magnesium can reduce the number, size and distribution of inclusions in steel, and improve the morphology; a small amount of magnesium can improve the size and distribution of carbides in stainless steel, and the formed MgO inclusions have the effect of pinning austenite grain boundaries, which has a good control effect on grain size; therefore, the mass content of Mg in the application is 0.0014-0.0199%.

[0025] The beneficial effects of the application are:

[0026] (1) The high-strength steel plate of the present application adopts a cheap chemical composition design of low carbon, low silicon and medium manganese, and does not contain precious metal elements such as Cr, Ni and Cu, thereby greatly reducing the material cost; the present application does not adopt the traditional Al deoxidization technology, but instead adopts Si-Mn primary deoxidization and Zr-Mg final composite deoxidization to form fine, dispersed and uniform composite oxysulfides, thereby greatly improving the plasticity and toughness and significantly improving the fatigue resistance.

[0027] (2) The high-strength steel of the present application is particularly suitable for offshore structures such as offshore platforms, ships and vessels, offshore wind tower drums, floating platform tower drums, floating body platform tower drums and support structures, which have high requirements for fatigue resistance.

[0028] On the basis of the above technical solution, the present application can be further improved as follows.

[0029] Further, it is composed of the following chemical components in terms of mass percentage: C: 0.039-0.069%, Si: 0.11-0.28%, Mn: 1.59-1.67%, Nb: 0.021-0.028%, Zr: 0.0059-0.0089%, Mg: 0.0031-0.0051%, and the rest is Fe and inevitable impurities.

[0030] Further, it is composed of the following chemical components in terms of mass percentage: C: 0.051%, Si: 0.19%, Mn: 1.55%, Nb: 0.027%, Zr: 0.0079%, Mg: 0.0041%, and the rest is Fe and inevitable impurities.

[0031] Further, in the inevitable impurities, the mass percentage of H element satisfies: H≤0.00019.

[0032] Further, under the conditions of low superheat pouring of 5-15℃ and electromagnetic stirring in the secondary cooling section, the center segregation of the high-strength steel continuous casting billet is below C level.

[0033] Center segregation refers to the phenomenon that the contents of carbon, phosphorus, sulfur, manganese and other elements in the center part of the continuous casting billet are higher than those in the edge of the billet. The evaluation standard adopted by the present application is: A, B, C and D four levels are used to evaluate according to the segregation morphology and peripheral contour, and the center segregation gradually weakens from A, B, C and D.

[0034] Further, the inclusions in the high-strength steel are all spherical fine inclusions, and the sum of the levels of A, B, C and D in the inclusions is less than or equal to 2.5 levels.

[0035] According to the JK table in GB / T10561-2005 "Microscope Evaluation Method of Non-metallic Inclusions in Steel", the shape and distribution of non-metallic inclusions are divided into four basic types: A type is sulfide type, B type is oxide type, C type is silicate type, and D type is spherical oxide type. Each type is divided into coarse and fine series according to different thickness or diameter, and each series is composed of 1-5 grades.

[0036] The type of the non-aqueous solution electrolytic extraction inclusions of the present application is a composite oxysulfide: the core of the inclusions is a Zr, Mg composite oxide, and the surface of the spherical particles is a sulfide (MnS).

[0037] Further, the high-strength steel plate of the present application still does not break under the fatigue peak stress of 415-475 MPa for 10 7 times, and the CTOD value (δm) of the high-strength steel at a temperature of -120℃ is 1.4-1.6 mm.

[0038] The beneficial effect of the above further scheme is that the above indicates that the fatigue resistance and fracture toughness of the high-strength steel of the present application are excellent.

[0039] Further, the high-strength steel is completely crack-arrested at a temperature of -10℃, and the crack-arrest toughness of the high-strength steel at a temperature of -20℃ is greater than or equal to 5400 N / mm 3 / 2 .

[0040] The beneficial effect of the above further scheme is that the above indicates that the crack-arrest performance of the high-strength steel of the present application is excellent.

[0041] The second aspect of the present application provides a method for preparing a high-strength steel with fatigue resistance and crack-arrest, comprising the following steps:

[0042] 1) sequentially performing smelting and refining on molten steel, then performing vacuum treatment, and then continuously casting into a billet to obtain a cast billet;

[0043] 2) performing conventional heating and soaking on the cast billet to obtain a heat-treated cast billet;

[0044] 3) continuously performing rolling on the heat-treated cast billet, and controlling the temperature of the final rolling to be 750-850℃, then water cooling to 410-550℃ after rolling, and then naturally cooling to room temperature to obtain a high-strength steel.

[0045] Further, the specific method of smelting and refining in step 1) is as follows: adopting a converter or an electric arc furnace, after the molten iron and / or scrap steel is smelted, the temperature and composition are adjusted to obtain molten steel, the temperature of the molten steel is adjusted to 1580-1670℃, and the free oxygen content in the molten steel is 121-389ppm; the molten steel is put into a ladle, and under the micro-sub-bubble stirring, the molten steel is pre-deoxidized by using Fe-Si alloy or Fe-Si-Mn alloy, and the free oxygen content in the molten steel is adjusted to 31-89ppm; under the micro-sub-bubble stirring, the molten steel is finally deoxidized by using a composite additive, and then the molten steel after the final deoxidization is subjected to LF refining, VD refining, or RH refining.

[0046] The composite additive is added into the molten steel in the form of a block alloy or a cored wire, the particle size of the composite additive is 5-15mm, the adding amount of the composite additive is 0.61-3.9kg per ton of molten steel, then the molten steel is subjected to LF refining, VD refining, or RH refining according to the conventional process, and finally the refined molten steel is subjected to continuous casting according to the conventional process; the composite additive is a combination of zirconium and magnesium, and the weight ratio of the zirconium and the magnesium is 2. BRIEF DESCRIPTION OF DRAWINGS

[0047] Figure 1 The size of the axial stress fatigue specimen of the present application;

[0048] Figure 2 The size and processing requirements of the CTOD specimen of the present application;

[0049] Figure 3 The size and processing requirements of the double tensile specimen of the present application;

[0050] Figure 4 The fatigue performance comparison of the high-strength steel prepared in Example 1 of the present application and the comparative steel;

[0051] Figure 5 The macro and micro morphologies of the fatigue fracture of the high-strength steel prepared in Example 1 of the present application under peak stress; wherein (a) is 400MPa, (b) is 450MPa, (c) is 475MPa, (d) is 400MPa, (e) is 450MPa, and (f) is 475MPa;

[0052] Figure 6 The macroscopic morphology comparison of the CTOD specimen fracture of the high-strength steel prepared in Example 1 of the present application and the comparative steel at-100℃;

[0053] Figure 7 The morphology of inclusions in the high-strength steel of the present application which resists fatigue and crack arrest;

[0054] Figure 8 The element distribution of inclusions in the high-strength steel of the present application which resists fatigue and crack arrest. DETAILED DESCRIPTION

[0055] The principles and features of the present application are described below, and the examples are used only to explain the present application and not to limit the scope of the present application. If a specific technique or condition is not mentioned in the examples, the technique or condition described in the literature in the art or according to the product manual is used. If the manufacturer of the reagent or instrument is not mentioned, it is a conventional product that can be purchased through a regular channel.

[0056] The composite additive in the following is a combination of zirconium and magnesium with a weight ratio of 2; the comparative high-strength steel (C 0.05%, Si 0.2%, Mn 1.5%, Nb 0.03%, Al 0.04%) is subjected to final deoxidization using a conventional aluminum block, aluminum particle or aluminum wire to form coarse and clustered aluminum oxide and composite oxides thereof.

[0057] Example 1

[0058] The present example relates to a high-strength steel resistant to fatigue and crack, which is composed of the following chemical components in mass percentage: C 0.051%, Si 0.19%, Mn 1.55%, Nb 0.027%, Zr 0.0079%, Mg 0.0041%, H 0.00018%, and the balance of Fe and inevitable impurities.

[0059] The present application relates to a method for preparing a high-strength steel resistant to fatigue and crack, which comprises the following steps:

[0060] 1) sequentially performing smelting and refining on the molten steel, then performing vacuum treatment, and finally continuously casting into a billet to obtain a cast billet;

[0061] The specific method for smelting and refining in step 1) is as follows: using a converter, adjusting the temperature and composition of the molten steel after steelmaking, adjusting the tapping temperature to 1610°C, and adjusting the free oxygen content in the molten steel to 255 ppm; after the molten steel enters the ladle, using micro-sub-bubble stirring for 8 minutes, then pre-deoxidizing in the ladle using Fe-Si alloy to adjust the free oxygen content in the molten steel to 60 ppm, stirring for 5 minutes using micro-sub-bubble, and then performing final deoxidization using a composite additive; the composite additive is added to the molten steel in the form of a block alloy, the particle size of the composite additive is 10 mm, the addition amount of the composite additive is 2.4 kg per ton of molten steel, and then the molten steel is subjected to LF and RH refining.

[0062] LF refining:

[0063] The viscosity of the refining slag is controlled at 1.49-1.99 Pa.s, so as to improve the ability of the slag system to absorb inclusions, thereby improving the cleanliness of the molten steel; the white slag basicity of the refining furnace is controlled at 4.9≤R≤7.9, which is beneficial to improve the desulfurization rate and improve the cleanliness of the molten steel and reduce the oxide inclusions in the molten steel; the MI slag index (=CaO / SiO2:Al2O3 ratio) MI>0.149, the sulfur partition coefficient is greatly increased, so as to control the appropriate fluidity of the refining slag at a certain basicity; the white slag retention time is ≥14.9 min, the refining period is ≥39.9 min, and the soft blowing time is >4.9 min, so as to control the [O] content of the outgoing.

[0064] RH vacuum treatment:

[0065] The vacuum chamber air pressure is extracted to below 66.7 kPa and maintained for 11.9-14.9 min, and the bottom blowing argon flow rate is 9.9-19.9 m 3 / h, so as to realize 5 times of molten steel circulation; the type and weight of the added alloy are strictly controlled, and higher-grade low-carbon ferromanganese, metallic manganese, low-carbon ferrosilicon, ferrotitanium and other alloys are used to ensure that the molten steel composition is completely qualified and the molten steel is more pure after the alloy is added and maintained in vacuum for more than 4.9 min; at the same time, the molten steel temperature is suitable for continuous casting, and the tundish superheat is controlled at 9.9-29.9 ℃ above the liquidus.

[0066] Then the refined molten steel is continuously cast.

[0067] The continuous casting process in step 1) is as follows: the molten steel superheat is controlled at 10 ℃, and electromagnetic stirring is used in the secondary cooling section. Through the above control measures, the center segregation of the continuous casting billet is C level 0.5.

[0068] 2) The billet is heated and soaked to obtain a heat-treated billet;

[0069] 3) The heat-treated billet is continuously rolled, and the temperature of the final rolling is controlled at 750-850 ℃, and then water cooling is performed after rolling to 410-550 ℃, and then natural cooling is performed to room temperature to obtain a high-strength steel.

[0070] The rolling process in step 3) is as follows: the billet is heated and soaked at a temperature of 1180 ℃, and then continuously rolled into a steel plate, with the final rolling temperature controlled at 810 ℃, and then water cooling is performed after rolling to 490 ℃, and then natural cooling is performed to room temperature.

[0071] The inclusions in the steel plate prepared by the above method are all spherical fine inclusions, and A+B+C+D≤2.5; the steel plate has a fatigue peak stress of 440 MPa and a cycle of 10 7The secondary still has not been broken; the fracture toughness of the steel plate material is excellent - the CTOD value (δm) at -120℃ is 1.51 mm; the steel plate is completely crack-arrested at -10℃, and the crack-arrest toughness at -20℃ is 5407 N / mm 3 / 2 .

[0072] Example 2

[0073] This example relates to a high-strength fatigue-resistant and crack-arrested steel consisting of the following chemical composition in mass percentage: C 0.069%, Si 0.28%, Mn 1.67%, Nb 0.028%, Zr 0.0089%, Mg 0.0031%, H 0.00019%, the balance being Fe and inevitable impurities.

[0074] The present application relates to a method for preparing a high-strength fatigue-resistant and crack-arrested steel, comprising the following steps:

[0075] 1) sequentially performing smelting and refining on the molten steel, then performing vacuum treatment, and finally continuously casting into a billet to obtain a cast billet;

[0076] The specific method for smelting and refining in step 1) is as follows: after the molten iron is smelted by a converter, the temperature and composition of the molten steel are adjusted, the tapping temperature is adjusted to 1670℃, and the free oxygen content in the molten steel is 389 ppm; after the molten steel enters a ladle, micro-subbubble stirring is performed for 10 minutes, then pre-deoxidation is performed in the ladle by using Fe-Si alloy or Fe-Si-Mn alloy to adjust the free oxygen content in the molten steel to 89 ppm, micro-subbubble stirring is performed for 5 minutes, and then final deoxidation is performed by using a composite additive; the composite additive is added to the molten steel in the form of a block alloy, the particle size of the composite additive is 14 mm, the addition amount of the composite additive is 3.7 kg per ton of molten steel, and then the molten steel is subjected to LF refining and RH refining.

[0077] LF refining:

[0078] The viscosity of the refining slag is controlled to be 1.45-1.95 Pa·s to improve the ability of the slag system to absorb inclusions, thereby improving the cleanliness of the molten steel; the basicity of the white slag of the refining furnace is controlled to be 5.1≤R≤7.5, which is conducive to improving the desulfurization rate and improving the cleanliness of the molten steel and reducing the oxide inclusions in the molten steel; the MI slag index (=CaO / SiO2:Al2O3 ratio) MI>0.141, the partition coefficient of sulfur has a greater increase, thereby controlling the appropriate fluidity of the refining furnace slag under a certain basicity; the white slag retention time is ≥14.1 min, the refining period is ≥39.1 min, and the soft blowing time is >4.9 min, thereby controlling the [O] content of the outgoing molten steel.

[0079] RH vacuum treatment:

[0080] The vacuum chamber is pumped to a pressure of 66.7 kPa or less and maintained for 12.1-14.8 min, and the argon flow rate of the bottom blowing is 10.1-19.5 m 3 / h, so that the molten steel is circulated 6 times; the types and weights of the added alloys are strictly controlled, higher-grade low-carbon ferromanganese, metallic manganese, low-carbon ferrosilicon, ferrotitanium and other alloys are used to ensure that the composition of the molten steel is completely qualified and the molten steel is more pure after the alloys are added and the vacuum is maintained for 5.1 min or more; and the molten steel is provided with a suitable temperature for continuous casting, so that the overheat of the tundish is 10.1-29.5 ℃ above the liquidus.

[0081] Then the refined molten steel is continuously cast according to a conventional process.

[0082] The continuous casting process in step 1) is as follows: the overheat of the molten steel is controlled to be 15 ℃, and electromagnetic stirring is used in the secondary cooling zone. Through the above control measures, the center segregation of the continuous casting billet is C level 0.5.

[0083] 2) The continuous casting billet is heated and soaked according to a conventional process to obtain a heat-treated continuous casting billet;

[0084] 3) The heat-treated continuous casting billet is continuously rolled, and the temperature of the final rolling is controlled to be 750-850 ℃, and then water cooling is performed to 410-550 ℃ after rolling, and then natural cooling is performed to room temperature to obtain a high-strength steel.

[0085] The rolling process in step 3) is as follows: the continuous casting billet is heated and soaked, and the heating and soaking temperature is 1210 ℃; then the continuous casting billet is continuously rolled into a steel plate, and the final rolling temperature is controlled to be 840 ℃, and then water cooling is performed to 530 ℃ after rolling; and then natural cooling is performed to room temperature.

[0086] The inclusions in the steel plate prepared by the above method are all spherical fine inclusions, and A+B+C+D≤2.5; the steel plate is not broken after 10 7 times of cyclic loading at a fatigue peak stress of 470 MPa; the fracture toughness of the steel plate material is excellent, and the CTOD value δm at -120 ℃ is 1.58 mm; the steel plate is completely stopped at -10 ℃, and the stop cracking toughness at -20 ℃ is 5413 N / mm 3 / 2 .

[0087] Example 3

[0088] This embodiment relates to a high-strength steel resistant to fatigue and cracking, which is composed of the following chemical components in terms of mass percentage: C 0.039%, Si 0.11%, Mn 1.59%, Nb 0.021%, Zr 0.0059%, Mg 0.0051%, H 0.00017%, and the balance being Fe and inevitable impurities.

[0089] The present application relates to a method for preparing a high-strength steel resistant to fatigue and cracking, which comprises the following steps:

[0090] 1) smelting and refining the molten steel in sequence, vacuum treatment, and then continuous casting to obtain a cast slab;

[0091] The smelting and refining method in step 1) is as follows: using an electric arc furnace, after the scrap steel is smelted, the temperature and composition of the molten steel are adjusted, the tapping temperature is adjusted to 1630°C, and the free oxygen content in the molten steel is 125 ppm; after the molten steel enters the ladle, it is stirred for 6 minutes using micro-sub-bubble stirring, then the free oxygen content in the molten steel is adjusted to 37 ppm by pre-deoxidation using Fe-Si-Mn alloy in the ladle, and after micro-sub-bubble stirring for 5 minutes, final deoxidation is performed using a composite additive; the composite additive is added to the molten steel in the form of a cored wire, the particle size of the composite additive in the cored wire is 1.5 mm; the addition amount of the composite additive is 0.69 kg per ton of molten steel, then the molten steel is refined by LF and VD according to the conventional process; finally, the refined molten steel is continuously cast according to the conventional process.

[0092] The continuous casting process in step 1) is as follows: the superheat of the molten steel is controlled at 9°C, electromagnetic stirring is used in the secondary cooling zone, and through the above control measures, the center segregation of the continuous casting slab of the steel plate is C level 0.5.

[0093] 2) heating and soaking the cast slab according to the conventional process to obtain a heat-treated cast slab;

[0094] 3) continuously rolling the heat-treated cast slab, and controlling the temperature of the final rolling to be 750-850°C, then water cooling to 410-550°C after rolling, and then naturally cooling to room temperature to obtain a high-strength steel.

[0095] The rolling process in step 3) is as follows: the cast slab is heated and soaked at a temperature of 1190°C; then continuously rolled into a steel plate, with the final rolling temperature controlled at 770°C, and then water cooled to 460°C after rolling; and then naturally cooled to room temperature.

[0096] The inclusions in the steel plate prepared by the above method are all spherical fine inclusions, and A+B+C+D≤2.5; the steel plate does not break after 10 7 cycles at a fatigue peak stress of 420 MPa; the fracture toughness of the steel plate material is excellent, with a CTOD value δm of -120°C of 1.47 mm; the steel plate completely stops cracking at -10°C, and the cracking toughness at -20°C is 5387 N / mm 3 / 2 .

[0097] Experimental Example

[0098] The high-strength steel prepared in Example 1 (hereinafter referred to as the high-strength steel prepared in Example 1 of the present application) was subjected to the following experiments.

[0099] 1. Test method

[0100] (1) Fatigue SN curve test

[0101] Fatigue testing was conducted in accordance with GB / T3075-2021 "Methods for Controlling Axial Force in Fatigue Testing of Metallic Materials". Specimen dimensions are as follows: Figure 1 As shown, the surface of the working section was polished to a bright finish along the axial direction using 3000# sandpaper. The test was a tensile-tensile fatigue test, stress-controlled, with a stress ratio of 0.1 and a loading frequency of 10Hz. The method for determining the SN curve is as follows: First, the fatigue limit was determined using the rise-fall method: the stress level applied in the first fatigue test was the estimated fatigue limit, i.e., the material yield strength × 0.9. Based on the fatigue results, the stress level for the next test was determined, i.e., the stress level was set after more than 10 cycles. 7 If the test is considered passed, the stress level is increased by 15 / n MPa; otherwise, it is decreased by 15 / n MPa (n is the number of fatigue tests). Generally, n is 5. The fatigue limit is determined based on the data distribution results. Then, a stress level is set every 25 MPa between the fatigue limit and the tensile strength for fatigue testing. Three sets of parallel samples are completed for each stress level.

[0102] (2) Low-temperature CTOD test

[0103] The low-temperature CTOD test shall be performed in accordance with GB / T 21143-2014 "Unified test method for the determination of quasistatic fracture toughness" or ISO 12135:2021 "Metallic materials - Unified method of test for the determination of quasistatic fracture toughness". Specimen dimensions and processing requirements are as follows: Figure 2 As shown. Before the CTOD test, a pre-crack was created, and the parameters of the pre-crack fatigue were as follows: the maximum stress intensity factor K in the final stage. fmax 38.0 MPa·m 1 / 2 The frequency was 8 Hz, the stress ratio was 0.1, the waveform was sinusoidal, and the span was 96 mm. The CTOD test loading rate was 1.0 mm / min, the loading time was 1.5-5.5 min, and the initial stress intensity factor growth rate was 1.8-2.1 MPa·m. 1 / 2 / s, with test temperatures of -100 and -120℃.

[0104] (3) Low-temperature crack arrest toughness test

[0105] The low-temperature crack arrest toughness test was performed according to the summary requirements for isothermal crack arrest temperature tests in Annex 4 of the 2020 China Classification Society's "Inspection Guidelines for High-Strength Steel Plates for Marine Use". The double tensile method was used for brittle crack initiation. Specimen dimensions and processing requirements are as follows: Figure 3The secondary tensile plate loading rate is not particularly required, as long as the brittle crack initiation enters the main plate. The main tensile plate is cooled, and when the temperature of each region reaches the target temperature T target After holding for 10 min, the tensile stress is applied to the main tensile plate, and the stress range is 0.45-0.6σ p0.2 After holding for at least 30 s, the secondary tensile plate is started until the crack initiation. The brittle crack initiation enters the main tensile plate and stops, and the crack length is recorded. The stop crack toughness at the target temperature is calculated by formula (1):

[0106]

[0107] In the formula, R is the main tensile stress, a is the crack stop length, and W is the sample width. If the crack does not stop, the sample is considered not to stop at the target temperature, and the stop crack toughness cannot be calculated.

[0108] (4) Inclusion extraction test

[0109] The extraction of inclusions uses a non-aqueous solution electrolysis technology, and the specific separation method is as follows: 500 mL of electrolyte solution is configured, wherein 5.0 g of tetramethylammonium chloride, 3.0 g of anhydrous barium oxide, 50 mL of acetylacetone and 450 mL of anhydrous methanol are added. In the non-aqueous solution (pH≈8) at 0-5℃, electrolysis is carried out to separate the inclusions from the iron matrix for morphology observation and composition analysis. The extracted inclusions are scanned by a scanning electron microscope (SEM, FEI Nova 400) equipped with an energy dispersive spectrometer (EDS) to determine the chemical composition of the inclusions and observe the morphology, particle size analysis, etc.

[0110] 2. Test results

[0111] (1) Fatigue S-N curve

[0112] Figure 4 The fatigue performance of the high-strength steel prepared in Example 1 is compared with that of the same type of control steel. As can be seen from the figure, at the same stress level, the fatigue life of the high-strength steel prepared in Example 1 is close to or exceeds 10 7 , while the fatigue life of the control steel is in the order of 10 5 ; at the same fatigue life, the fatigue strength that the high-strength steel prepared in Example 1 can withstand is 25-50 MPa higher than that of the control steel.

[0113] Figure 5The macro and micro morphology of fatigue fracture of the high-strength steel prepared in Example 1 under different peak stress levels. At 400 MPa and 450 MPa, the fatigue fracture has the characteristics of typical crack initiation, stable propagation, unstable propagation and instantaneous fracture shear lip, and the quasi-cleavage morphology of river-like pattern (with cleavage facets and tear ridges) at the crack source; while at 475 MPa, the fatigue fracture shows the typical characteristics of positive tensile dimple fracture.

[0114] (2) Low-temperature CTOD value

[0115] Table 1 is the CTOD value of the high-strength steel prepared in Example 1 of the present application and the same type of comparative steel measured at -100℃. As shown in Table 1, the δ m of the high-strength steel prepared in Example 1 of the present application is 1.60 and 1.63 mm, and the δ m of the two comparative steels is 0.10 and 0.07 mm. As can be seen, the CTOD value of the high-strength steel prepared in Example 1 of the present application is much larger than that of the comparative steels.

[0116] Table 2 is the CTOD value of the high-strength steel prepared in Example 1 of the present application and the same type of comparative steel measured at -120℃. As shown in Table 2, the δ m of the high-strength steel prepared in Example 1 of the present application is 1.62 and 1.60 mm, and the δ c of the two comparative steels is 0.02 and 0.02 mm. As can be seen, the CTOD value of the high-strength steel prepared in Example 1 of the present application is much larger than that of the comparative steels.

[0117] Table 1 is the CTOD value of the high-strength steel prepared in Example 1 of the present application and the same type of comparative steel measured at -100℃

[0118]

[0119] Table 2 is the CTOD value of the high-strength steel prepared in Example 1 of the present application and the same type of comparative steel measured at -120℃

[0120]

[0121]

[0122] Figure 6 The macro morphology of the CTOD sample fracture of the high-strength steel prepared in Example 1 of the present application and the comparative steel at -100℃ and -120℃. Among them Figure 6 1, 2 represent two groups of parallel tests.

[0123] The fracture of the CTOD specimen of the high-strength steel prepared in Example 1 exhibited a ductile fracture feature at both -100°C and -120°C, with significant necking (extrusion in an arc shape from both sides to the middle) and a fracture surface with a high middle and low ends. The control steel exhibited a typical brittle fracture feature, with no necking deformation and a flat fracture surface.

[0124] From the comparison of Tables 1 and 2 and Figure 6 It can be seen from the comparison that the CTOD value of the high-strength steel prepared in Example 1 is not less than 1.4 mm, while the CTOD value of the control steel is not more than 0.1 mm. The difference of more than ten times between the two reflects the difference in fracture mechanism between the high-strength steel prepared in Example 1 and the control steel, which is ductile fracture for the former and brittle fracture for the latter.

[0125] (3) Low-temperature crack arrest toughness

[0126] The crack arrest of the double tensile test at -10°C showed that the crack of the high-strength steel prepared in Example 1 was arrested at the initial stage of entering the main plate at -10°C, indicating that the high-strength steel prepared in Example 1 could be completely arrested at -10°C. Table 3 shows the comparison of the crack arrest toughness of the high-strength steel prepared in Example 1 and the control steel. As shown in Table 3, the crack arrest toughness of the high-strength steel prepared in Example 1 at -20°C was 5409 N / mm 3 / 2 , while that of the control steel was 3000 and 4877 MPa. The high-strength steel prepared in Example 1 exhibited excellent crack arrest toughness compared with the control steel.

[0127] Table 3 Comparison of crack arrest toughness of high-strength steel prepared in Example 1 and control steel

[0128]

[0129]

[0130] Note: Reference [1]: Yuki Nishizono, Tomoya Kawabata, Shuji Aihara, Teppei Okawa, A simplified method for evaluation of brittle crack arrest toughness of steels in scaled-down bending tests, Engineering Fracture Mechanics, Volume 215, 2019, Pages 99-111.

[0131] (4) Inclusion characteristics

[0132] The Zr-Mg composite inclusion is obtained by using non-aqueous solution electrolysis technology. Figure 7 and 8 The figure and element distribution diagram of the inclusion in the high-strength steel with excellent anti-fatigue and crack arrest performance obtained by non-aqueous solution electrolysis extraction are shown in the following figures. Figure 7 and 8 It can be seen from the figures that the inclusion is spherical. The type of the inclusion extracted by non-aqueous solution electrolysis is composite oxysulfide: the core of the inclusion is Zr-Mg composite oxide, and the surface of the spherical particle is sulfide (MnS).

[0133] The type and size of the inclusion in the high-strength steel with excellent anti-fatigue and crack arrest performance are compared by using a scanning electron microscope. In the conventional Al deoxidized steel, the oxide inclusion is cluster-shaped, and the inclusion is mainly concentrated in 2-5 mu m. The sulfide is strip-shaped, and the length is usually 5-20 mu m. After Zr-Mg composite deoxidation treatment, the size of the inclusion is obviously refined, and is mainly concentrated in 1-3 mu m.

[0134] In summary, the steel plate of the present application adopts a cheap chemical component design of low carbon, low silicon and medium manganese, and does not contain precious metal elements such as Cr, Ni and Cu, thereby greatly reducing the material cost. The present application does not use the traditional Al deoxidation technology, but uses Si-Mn initial deoxidation and Zr-Mg final composite deoxidation to form fine, dispersed and uniform composite oxysulfide, thereby greatly improving the plasticity and toughness and significantly improving the anti-fatigue performance. The high-strength steel is especially suitable for offshore structures such as offshore platforms, warships and ships, offshore wind tower drums, floating platform tower drums, floating body platform tower drums and support structures, and has excellent anti-fatigue performance.

[0135] In the description of the present specification, the description of the terms "one embodiment", "some embodiments", "example", "specific example" or "some examples" means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, the person skilled in the art can combine and combine the different embodiments or examples described in the present specification and the features of the different embodiments or examples without contradiction.

[0136] Although the embodiments of the present application have been shown and described above, it can be understood that the above embodiments are exemplary and cannot be understood as limiting the present application, and the person skilled in the art can make changes, modifications, replacements and variations to the above embodiments within the scope of the present application.

Claims

1. A high-strength steel resistant to fatigue and cracking, characterized in that, It is composed of the following chemical components in mass percentage: C: 0.029-0.079%, Si: 0.09-0.29%, Mn: 1.41-1.69%, Nb: 0.019-0.029%, Zr: 0.0041-0.0199%, Mg: 0.0014-0.0199%, the rest being Fe and inevitable impurities; the mass percentage of Zr element and Mg element also satisfies the formula: 0.0041%<Zr+Mg<0.0199%, and Zr / Mg=1-3; Under the conditions of low superheat pouring of 5-15 ℃ and electromagnetic stirring in the second cooling section, the center segregation of the high-strength steel continuous casting billet is below C level; The inclusions in the high-strength steel are all spherical fine inclusions, and the sum of the levels of A, B, C and D in the inclusions is less than or equal to 2.5 levels; The CTOD value of the high-strength steel at a temperature of-120 ℃ is 1.4-1.6 mm. The high-strength steel is completely crack-arrested at a temperature of -10 DEG C; the high-strength steel is crack-arrested at a temperature of -20 DEG C with a crack-arrest toughness greater than or equal to 5400 N / mm 3 / 2 .

2. The high-strength fatigue and crack arrest resistant steel according to claim 1, characterized in that, It is composed of the following chemical components in mass percentage: C: 0.039-0.069%, Si: 0.11-0.28%, Mn: 1.59-1.67%, Nb: 0.021-0.028%, Zr: 0.0059-0.0089%, Mg: 0.0031-0.0051%, the rest being Fe and inevitable impurities.

3. The high-strength fatigue and crack arrest resistant steel of claim 1, wherein, It is composed of the following chemical components in mass percentage: C: 0.051%, Si: 0.19%, Mn: 1.55%, Nb: 0.027%, Zr: 0.0079%, Mg: 0.0041%, the rest being Fe and inevitable impurities.

4. The high-strength fatigue- and crack-arrest-resistant steel according to any one of claims 1 to 3, characterized in that, In the inevitable impurities, the mass percentage of H element satisfies: H≤0.00019.

5. A method of producing a high-strength steel according to any one of claims 1 to 4, characterized in that, It comprises the following steps: 1) sequentially performing smelting and refining on the molten steel, then performing vacuum treatment, and then continuously casting into a billet to obtain a billet; 2) heating and soaking the billet to obtain a heat-treated billet; 3) continuously rolling the heat-treated billet, controlling the temperature of the final rolling to be 750-850 ℃, water cooling to 410-550 ℃ after rolling, and then naturally cooling to room temperature to obtain a high-strength steel.

6. The method of claim 5, wherein the high-strength steel is prepared by the steps of: The specific method of smelting and refining in step 1) is: using a converter or an electric arc furnace, adjusting the temperature and composition after steelmaking of molten iron and / or scrap steel to obtain molten steel, adjusting the temperature of the molten steel to be 1580-1670 ℃, and the free oxygen content in the molten steel being 121-389 ppm; making the molten steel enter a ladle, pre-deoxidizing the molten steel in the ladle using Fe-Si alloy or Fe-Si-Mn alloy under micro-sub-bubble stirring to adjust the free oxygen content in the molten steel to be 31-89 ppm; finally deoxidizing the molten steel using a composite additive under micro-sub-bubble stirring, and then performing LF refining, VD refining or RH refining on the finally deoxidized molten steel. ​

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