WO2023241471A1

WO2023241471A1 - Anti-delayed cracking and wear-resistant steel plate and manufacturing method therefor

Info

Publication number: WO2023241471A1
Application number: PCT/CN2023/099335
Authority: WO
Inventors: 宋凤明; 温东辉; 陆敏; 华骏山; 张国民
Original assignee: 宝山钢铁股份有限公司
Priority date: 2022-06-15
Filing date: 2023-06-09
Publication date: 2023-12-21
Also published as: CN117265432A

Abstract

An anti-delayed cracking and wear-resistant steel plate and a manufacturing method therefor. The steel plate comprises the following chemical elements in wt%: C: 0.17-0.22%, Si: 0.1-0.3%, Mn: 1.0-1.4%, P≤0.015%, S≤0.005%, Al: 0.018-0.04%, Cu: 0.15-0.60%, Ni: 0.1-0.31%, B: 0.001-0.003%, N≤0.005%, and one or two among Nb: 0.01-0.03% and Ti: 0.01-0.03%, the balance being Fe and unavoidable impurities; and the steel plate satisfies: 5.68N≤Nb+Ti≤0.044 and Cu/Ni≤2.0. The steel plate of the present application shows excellent anti-delayed cracking properties and is suitable for manufacturing pipelines in areas such as land reclamation and waterway dredging.

Description

Delayed cracking-resistant and wear-resistant steel plate and manufacturing method thereof

Technical field

The present invention relates to the field of alloys, and in particular to an anti-delayed cracking and anti-corrosion steel plate suitable for slurry dredging and a manufacturing method thereof.

Background technique

During land reclamation, channel dredging, bank maintenance and other operations, a large amount of solid particles such as sediment and gravel are transported over long distances in the form of slurry through dredging pipelines. The pipe body is subject to electrochemical corrosion of the slurry medium and abrasion of the solid particles. And the interaction between the two, especially when the seawater slurry contains weathered rocks, coral reefs, and medium-coarse sand, the abrasion on the inner wall of the pipe is more serious. Existing dredging pipelines are mostly made of ordinary Q235B and Q345B materials, which have a short service life under harsh working conditions and may even be scrapped in less than one year. Due to the interaction between corrosion and wear during the failure process, the material failure caused by abrasion is much higher than the sum of pure corrosion and wear. Therefore, the steel used for dredging pipes is required to not only have wear resistance, but also corrosion resistance, thus having good Abrasion resistance. In order to reduce dredging costs, it is required to use higher strength corrosion-resistant steel plates to make dredging pipes to increase pipeline life. Research shows that high-strength steel plates have delayed cracking problems in corrosive dredging conditions, so the wear-resistant steel used for high-strength dredging pipes must solve the delayed cracking problem.

In terms of improving the wear resistance of steel materials, there are currently many related existing technologies disclosed. For example, CN103397272A discloses "a wear-resistant steel plate with low crack sensitivity index and high strength and its preparation method" and CN103103448A discloses "a low-alloy high-strength and toughness wear-resistant steel plate". These two patents involve steel types with a hardness of 450HBW, which are mainly used in engineering machinery, mining equipment and other fields and have good wear resistance. The composition design is based on C-Mn and a higher Mo alloy element is added, so the alloy cost is high. Steel contains high corrosion resistance element Si, which is detrimental to toughness. The above-mentioned patented steel types do not take measures to control corrosion and cannot meet the usage requirements under corrosion + wear conditions.

A large number of patents have also been applied for and published abroad in the field of wear resistance, which are mainly used in engineering machinery manufacturing, but do not involve the field of slurry transportation with abrasive characteristics. like:

US5284529A discloses "Abrasion-resistant Steel", which involves steel containing up to 0.05-1.5% Ti and 0.1-3.0% Mo. The alloy cost is high and the maximum hardness is 420HBW.

JP2007231321A and JP2008169443A respectively disclose "Wear Resistant Steel Sheet" and "Wear-Resistant Steel Sheet Superior in Workability and Manufacturing Method Therefor", which introduce a method to improve wear resistance through carbide precipitation particles of Ti and W, but the hardness of the former Basically between 396-431HBW, the latter is less than 300HBW and cannot reach the 450HBW hardness level. A large number of carbide particles in its matrix act as a cathode in an abrasive environment, promoting the occurrence of electrochemical corrosion and increasing the abrasion weight loss of the material. Therefore, although the steel plate has good wear resistance, it has poor abrasion resistance and has no The issue of resistance to delayed cracking has been considered.

CN101886225A discloses "a corrosion-resistant and wear-resistant steel and a preparation method thereof". The patent involves a steel type with a hardness of 52HRC or above, with up to 0.4-0.9% C and 14-16% Mn added to the matrix, and the contents of Mo and Cr are equal. At 5-10%, it also contains a certain amount of rare elements such as Pr, Nd and Gd. It is a high-alloy steel and the cost is very high.

CN102776445A and CN108930001A respectively disclose "a lower bainite wear-resistant steel pipe for slurry transportation and its manufacturing method" and "a high-hardness wear-resistant steel plate for slurry dredging and its production method". The steel type involved in the former They are all bainite or bainite + acicular ferrite structure. The matrix hardness is not high and the tensile strength is only 600-800MPa. It is mainly used for the transportation of slurry or crude oil with small particles (tens of μm) and is not suitable for large-scale transportation. In the field of transporting granular and high-density seawater slurry; the latter is a 450HBW ultra-high-strength corrosion-resistant steel plate, and the problem of delayed cracking resistance is not considered in the composition design and performance requirements. During the dredging operation, the steel plate is prone to crack initiation when it is bumped or scratched by hard objects. Especially in corrosive environments, delayed cracking is prone to occur, resulting in leakage or even cracking of the pipe body during the dredging process, affecting the smooth progress of the dredging operation.

In dredging operations, the dredging pipeline, which is an important part, faces corrosion problems both inside and outside the pipe during use. The outer wall of the pipe inevitably withstands bumps and scratches from hard objects. When the strength of the pipe body steel plate is low, such as Q235B and 3Q345B, the low yield strength of the steel plate itself can deform and absorb the impact energy to ensure the safety of the pipe body; but for high-strength steel plates, especially ultra-high-strength steel plates with a yield exceeding 1000MPa , when subjected to such damage, the damage stress is difficult to exceed the yield strength of the steel plate and cannot deform, resulting in the initiation and expansion of cracks at the injured location. In a corrosive environment, the initiation and expansion of cracks promote the penetration and diffusion of hydrogen, especially electrochemical corrosion, which also promotes the precipitation and accumulation of hydrogen. Hydrogen atoms penetrate into the internal lattice of steel, causing the vacancy concentration to increase, thereby forming micropores of vacancy groups, further promoting the germination of microcracks, leading to brittle cracking of the steel plate, that is, delayed cracking. This will significantly affect the normal progress of dredging operations, shorten the service life of dredging pipelines, and increase dredging costs. The higher the strength of the steel plate, the more susceptible it is to hydrogen penetration. Therefore, under dredging conditions, even if there is no damage to the surface of the pipe body, there is still the problem of delayed cracking caused by hydrogen. The performance of high-strength steel plates used for dredging pipelines must consider their resistance to delayed cracking.

It can be found from the existing technology that the current wear-resistant steel either does not consider corrosion resistance, or does not consider the problem of delayed cracking under high stress, and is not suitable for the production and processing of dredging pipes.

Contents of the invention

The purpose of the present invention is to provide a delayed cracking-resistant and wear-resistant steel plate and a manufacturing method thereof, with a yield strength ≥1100MPa, a tensile strength ≥1300MPa, an elongation ≥12%, a hardness of 450±30HBW, and a -40°C impact power ≥ 60J, the wear resistance is twice that of ordinary steel plates. In the case of U-shaped bending, it will not crack when immersed in 0.1mol/L hydrochloric acid for more than 600 hours (that is, the cracking time is more than 600 hours). The steel plate applied for shows excellent delayed cracking resistance and is suitable for the production of pipelines in areas such as sea reclamation and waterway dredging. There is no risk of cracking and leakage when the surface is hit or scratched in a corrosive environment, thus greatly improving the Improve dredging efficiency and reduce operating costs.

In order to achieve the above goals, the present invention provides a steel plate with excellent delayed cracking resistance and abrasion resistance, the steel plate containing the following chemical elements in wt%: C: 0.17-0.22%, Si: 0.1-0.3% , Mn: 1.0-1.4%, P≤0.015%, S≤0.005%, Al: 0.018-0.04%, Cu: 0.15-0.60%, Ni: 0.1-0.31%, B: 0.001-0.003%, N≤0.005% , and one or both of Nb: 0.01-0.03% and Ti: 0.01-0.03%, the balance is Fe and inevitable impurities.

Preferably, the elements N, Nb and Ti satisfy the following inequality: 5.68N≤Nb+Ti≤0.044. In some embodiments, the lower limit of Nb+Ti in the above inequality may be, for example, 6.15N, 6.37N, 6.65N, etc. In some embodiments, the upper limit of Nb+Ti in the above inequality may be, for example, 0.044, 0.04, 0.039, 0.034, etc. For example, the elements N, Nb and Ti satisfy the following inequality: 6.65N≤Nb+Ti≤0.04.

Preferably, the elements Cu and Ni satisfy the following inequality: Cu/Ni≤2.0. The lower limit of Cu/Ni is not limited, and may be 0, 0.7 or 1.1, for example. The upper limit of Cu/Ni is not limited, and may be 2, 1.9, 1.8, 1.6 or 1.5, for example. For example, the elements Cu and Ni satisfy the following inequality: 0.7≤Cu/Ni≤2.0.

Preferably, the steel plate also contains Cr≤2.0%, W: 0.01-0.5%, Mo: 0.01-0.5%, Sb: 0.01-0.2%, REM: 0.01-0.2%, V: 0.01-0.2% and Ca: More than one of 0.001-0.01%.

Preferably, the Cu content in the steel plate is 0.29-0.60%.

Preferably, the thickness of the steel plate is 8-20mm.

In the composition design of the steel plate of the present invention:

C is the cheapest reinforcing element in steel and can significantly improve the strength of steel plates. However, more C is detrimental to the welding, toughness and plasticity of steel plates. The range is limited to 0.17-0.22% under the condition that the performance requirements are met.

Si is a deoxidizing element, a solid solution strengthening element, and a commonly used corrosion-resistant element in atmospheric corrosion-resistant steel. Si It replaces Fe atoms in steel by substitution, hindering dislocation movement to achieve solid solution strengthening. Si can reduce the diffusion coefficient of C in ferrite, increase the activity of carbon, inhibit the formation of carbides, and inhibit the precipitation of coarse carbides in defects to improve toughness. However, too high Si promotes the graphitization of C, which is detrimental to toughness; it is also detrimental to surface quality and welding performance. Therefore, its content is limited to 0.1-0.3%.

Mn is also a common strengthening element in steel. It improves the yield strength through solid solution strengthening, reduces the elongation, significantly reduces the phase transformation temperature of the steel, and refines the microstructure of the steel. It is an important strengthening and toughening element, but the Mn content is too high. It will increase the hardenability, which will lead to the deterioration of weldability and welding heat affected zone toughness and increase the cost. So control it between 1.0-1.4%.

P is the main corrosion-resistant element in traditional atmospheric corrosion-resistant steel. It promotes the formation of surface protective rust layer and effectively improves the atmospheric corrosion resistance of steel. However, the formation of surface rust layer during the abrasion process will accelerate the abrasion weight loss of the material and reduce the weight loss of the material. In terms of wear resistance, the presence of P can easily cause segregation, reduce the toughness and plasticity of steel, and make the steel plate brittle, affecting the toughness. Therefore, the content of P in the steel should be reduced as much as possible. In the present invention, its content is required to be controlled below 0.015%.

S can increase the yield strength of steel, but the presence of S will deteriorate the atmospheric corrosion resistance of steel and make the steel plate brittle, reducing the low-temperature toughness of steel. It is required to control its content below 0.005%.

Al is usually added to steel as a deoxidizer during the steelmaking process. Trace amounts of Al are beneficial to refining the grains and improving the strength and toughness of steel. Al is a ferrite-forming element. On the one hand, more Al reduces the strength of the steel plate and increases the brittleness of the ferrite in the steel, resulting in a decrease in the toughness of the steel. Therefore, its content is limited to 0.018-0.04%, preferably 0.02-0.04%.

Cu has solid solution and precipitation strengthening effects. When the content is high, tempering at an appropriate temperature has a secondary hardening effect, thereby improving the strength. Cu is also one of the elements that improves corrosion resistance. The electrochemical potential is higher than that of Fe. Adding an appropriate amount of Cu is conducive to increasing the self-corrosion potential of the steel plate itself and reducing the corrosion rate; it promotes the densification of the rust layer on the steel surface and stabilizes the rust layer. formation, thereby improving corrosion resistance. With the improvement of corrosion resistance, the precipitation of hydrogen during the corrosion process is reduced, and the resistance to delayed cracking is improved. Adding copper to steel can inhibit the diffusion of hydrogen and reduce the sensitivity to hydrogen-induced cracking, especially in combination with Cr. Improved delayed cracking resistance. In order to ensure the role of Cu, its content should not be less than 0.15%. Too high Cu will cause cracks in the steel billet during heating and hot rolling, deteriorating surface properties, and the upper limit is limited to 0.60%.

Ni exists in solid solution form in steel and does not form carbides. It is an element that forms expanded austenite. The addition of Ni has a grain refining effect. It can improve low-temperature impact toughness by refining grains and reducing stacking faults. In high-strength steel, nickel can also homogenize the structural structure of the steel, inhibit the diffusion behavior of hydrogen, and reduce the risk of irreversible hydrogen traps. content, thereby improving the delayed cracking resistance. Ni is also an important corrosion-resistant element enriched in the rust layer, which refines the grains of the rust layer and Promote the formation of nano-phase, superparamagnetic α-FeOOH in the inner rust layer, and the particle size of the formed α-FeOOH is less than 15nm, thus increasing the density of the inner rust layer, making it difficult for chloride ions to penetrate the rust layer and Steel substrate contact, thereby reducing the corrosion rate. In particular, Ni can promote the stability of the rust layer and improve the hot working brittleness problem caused by Cu. Considering the effect of Cu on increasing potential and the inhibitory effect of Cu and Ni on hydrogen diffusion, the present invention uses Cu and Ni as important elements to improve the resistance to delayed cracking. In order to achieve the best matching effect and consider the suppression of copper embrittlement, the content matching of Cu and Ni is limited, requiring Cu/Ni ≤ 2.0. However, Ni is a precious element, and the Ni content is limited to 0.1-0.31%, preferably 0.1-0.30%.

B is enriched in dislocations and defects in steel, reducing grain boundary energy and inhibiting ferrite transformation, so it has good hardenability, thereby increasing the hardness of the steel plate. In addition, trace amounts of B have a strong tendency to be enriched at austenite grain boundaries. The formed Fe ₂ B and austenite can form a better coherent interface, reducing the interface energy at the grain boundaries, thus delaying ferrite nucleation and improving Stability of austenite. The addition of B can improve the low-temperature impact toughness of the steel plate after low-temperature tempering and reduce the ductile-brittle transition temperature. The impact toughness of B-containing steel tempered at about 300°C is higher than that of B-free steel, and the impact toughness of B-containing steel tempered at temperatures above 500°C is lower than that of B-free steel. When the B content is low, it is enriched towards the half-atom plane of edge dislocations in the steel due to the hydrostatic pressure field of edge dislocations in austenite, reducing its impact on grain boundaries and its impact on hardenability. Not obvious. Therefore, the B content is required to be above 0.001%. Excessive B content will cause the grain boundary strength to decrease, and fracture and cleavage will occur along the grain when stressed, forming the phenomenon of "boron embrittlement". In addition, too high B is detrimental to welding, and the strengthening effect will not be further increased, and it is easy to segregate at grain boundaries to cause embrittlement and reduce stamping performance. Therefore, the B content is controlled below 0.003%.

N can form nitrides with Nb, V and Ti in steel. The fine precipitates have the effect of nailing the grain boundaries to refine the austenite grains. The precipitated nitrides have the effect of precipitation strengthening, but higher N AlN is easily formed when combined with Al in steel, thereby significantly increasing the number of nitrides in steel. When AlN exists independently in steel as a non-metallic inclusion, it destroys the continuity of the steel matrix. Especially when the Al content is high, the amount of AlN formed is large and aggregated, the degree of harm is even greater, and the formation of An oxide with poor plasticity; and higher N is easily enriched in defects, deteriorating low-temperature impact toughness. N, like C, tends to segregate at dislocations to form Coriolis air masses, leading to strain concentration. Therefore, N is controlled as an impurity element in the present invention, and the N content is limited to less than 0.0050%.

The addition of Ti and Nb causes N to form nitride, reducing the adverse effects of N. In order to further eliminate the adverse effects of N, it is preferred that the contents of elements N, Nb and Ti satisfy the relationship: 5.68N≤Nb+Ti≤0.044, preferably 6.65N≤Nb+Ti≤0.04.

Cu/Ni: Since the melting point of Cu is only about 1083°C, it is lower than the steel matrix. Excessive Cu will cause cracks in the steel billet during heating and hot rolling, causing copper brittleness and deteriorating surface properties; while the addition of an appropriate amount of Ni can inhibit copper brittleness caused by Cu and improve low-temperature impact toughness. Since Ni is a precious alloy element, too much Ni This will lead to an increase in manufacturing costs. Research shows that keeping the Cu/Ni ratio below 2.0 is enough to solve the copper embrittlement problem caused by the addition of Cu, so the ratio between the two is limited to ≤ 2.0.

Nb is a strong nitrogen carbide forming element and 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 and produce precipitation strengthening effects. Significantly improves the strength of the steel plate; and 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, so adding an appropriate amount of Nb to the steel is beneficial to the improvement of the strength. . The carbonitride formed by Nb can nail the austenite grain boundaries during the austenitization process, inhibit the abnormal growth of austenite grains, and help improve the toughness of the steel plate after quenching. However, more Nb is not good for welding. It easily forms brittle metal hydride with hydrogen. Its plastic toughness is greatly different from that of the matrix, and its bonding force with the matrix is also poor, resulting in delayed cracking. Recommended content is 0.01-0.03%.

On the one hand, Ti inhibits the growth of austenite grains during the reheating process of the slab, and inhibits the growth of ferrite grains during the recrystallization controlled rolling process, thereby improving the toughness of the steel. And Ti can preferentially combine with N in the steel to reduce the amount of AlN in the steel. However, too high Ti is detrimental to low-temperature impact toughness, and like Nb, it is easy to form brittle hydrides with hydrogen, which is detrimental to delayed cracking resistance. Therefore, 0.01-0.03% Ti is added.

Preferably, in order to further improve the performance, one or more of Cr, W, Mo, Sb, REM, V and Ca can be optionally added to the steel plate of the present invention, where Cr≤2.0%, W: 0.01-0.5 %, Mo: 0.01-0.5%, Sb: 0.01-0.2%, REM: 0.01-0.2%, V: 0.01-0.2% and Ca: 0.001-0.01%.

Cr is an important corrosion-resistant element and has a solid solution strengthening effect. At the same time, the addition of Cr can effectively increase the self-corrosion potential of steel and inhibit the occurrence of corrosion, thereby effectively reducing the promotion effect of corrosion on material failure during the abrasion process and improving wear resistance. Corrosion performance; especially with the improvement of corrosion resistance, the precipitation of hydrogen during the corrosion process can be reduced, thereby improving the delayed cracking resistance. However, Cr is a precious alloy element, and high Cr content promotes the formation of a protective rust layer on the steel surface. In an abrasive environment, these rust layers quickly detach from the surface, promoting the abrasive failure of the material. Therefore, we choose to add it and limit its content limit to 2.0%.

W forms carbides in steel to produce secondary strengthening and solid solution strengthening effects, and inhibits the segregation of impurity atoms and non-metallic inclusions at grain boundaries during over-aging to improve fracture toughness. Mo has phase transformation strengthening and dislocation strengthening effects, which can improve the tempering stability of steel, slow down the temper softening phenomenon, inhibit high-temperature temper brittleness, and improve the low-temperature impact toughness of steel plates. Sb can combine with Cu in steel to form a Cu2Sb film on the surface, thereby improving corrosion resistance. The addition of REM (rare earth metal) is conducive to the improvement of corrosion resistance. It forms REM compounds, REM/Fe intermetallic compounds and solid solution rare earths in steel, which are hydrolyzed in the corrosive thin liquid film and form in the cathode with a higher pH value. Precipitate, thus acting as a corrosion inhibitor. V is also a strong carbon-nitrogen compound-forming element and can precipitate during phase changes. It has solid solution strengthening and carbonitride precipitation strengthening effects in steel, and increases tempering stability, thereby improving strength. Adding Ca to steel can change the shape of the sulfide, inhibit the hot brittleness of S, and improve the toughness.

The steel type designed with the above ingredients not only has high strength and hardness, but also has a high self-corrosion potential, which inhibits the occurrence of corrosion and improves the wear resistance (the wear resistance is more than twice that of ordinary Q235B steel plates). After heat treatment, a high-strength martensite structure is obtained, with yield strength ≥1100MPa, tensile strength ≥1300MPa, elongation ≥12%, hardness 450±30HBW, and -40°C impact energy ≥60J. With excellent wear resistance and improved corrosion resistance, the steel type has good wear resistance. Through component design and performance optimization, good delayed cracking resistance (cracking time is more than 600h) is obtained. High-strength steel products made with it Dredging pipes are particularly suitable for transporting large particles and high-density slurry, and are not prone to cracking and leakage during use.

Further, this application provides a method for manufacturing the above-mentioned steel plate, which includes the following steps:

1) Smelting and casting

Smelting and casting molten steel to obtain a cast slab;

2) Heating of cast slab

The heating temperature is above 1230°C, and the total heating time of the slab in the heating furnace is not less than 2 hours, of which the soaking time in the soaking section is not less than 40 minutes;

3) Rough rolling and finish rolling

In the rough rolling stage, large reduction is used, and the reduction rate of each pass is controlled to be above 15%, and/or the reduction of each pass is above 25mm, and/or the total deformation ratio of each pass is greater than 80%. ;

In the finishing rolling stage, the reduction rate of the last pass is controlled to be no less than 16%, and the finishing rolling temperature is ≥880°C, preferably 880-898°C;

4) Cooling and coiling

Cooling adopts laminar flow cooling, cooling to 550-680℃, and then coiling;

5)Quenching and tempering

The quenching temperature is 820-845°C. The quenching holding time T1 starts from the center of the steel plate to the temperature. T1 is 1.5H-2H, unit: min, where H represents the plate thickness in mm; the steel plate is directly water-quenched to room temperature after it is released. , cooling rate ≥50℃/s;

The tempering temperature is 200-240°C, and the tempering holding time T2 starts from the center of the steel plate to the temperature. T2 is 2H-3H, unit: min, where H represents the plate thickness in mm, and T2 ≥ 12min;

6) Finishing treatment

Straightening and trimming.

Preferably, in step 1), the cast slab is hot-loaded into the furnace after the casting is completed, that is, after confirming that there are no quality problems on the surface of the cast slab, it is directly transported from the casting area to the heating furnace through the roller conveyor for heating and heat preservation, thereby reducing energy consumption; If hot charging is not possible, the cast slab must be placed in the heat preservation pit for slow cooling. The heat preservation pit can be removed for air cooling after the temperature drops below 200°C.

Preferably, in step 4), cool to 560-680°C and then coil.

Preferably, in step 5), the quenching temperature is 828-845°C.

Preferably, in step 5), the tempering temperature is 210-240°C, preferably 220-240°C.

Preferably, in step 5), the steel coil cooled to room temperature is uncoiled and straightened and then cut into plates, and then the steel plate is quenched and tempered.

Preferably, the thickness of the corrosion-resistant steel plate obtained is 8-20 mm.

In the manufacturing method of the steel plate of the present invention:

The cast slab is heated and kept warm before rolling, and the heating temperature is above 1230°C. The heating and heat preservation of the cast slab in the heating furnace is divided into a preheating section, a heating section and a soaking section. The present invention requires that the total heating time of the cast slab in the heating furnace is not less than 2 hours, and the heat preservation time of the soaking section is not less than 40 minutes. In addition, the cast slab can be hot loaded into the furnace after the casting is completed. That is, after confirming that there are no quality problems on the surface of the cast slab, it is directly transported from the casting area to the heating furnace through the roller conveyor for heating and heat preservation, thereby reducing energy consumption; if hot charging is not possible, then After casting, the cast slab must be placed in the heat preservation pit for slow cooling. The heat preservation pit can be removed for air cooling after the temperature drops below 200°C.

Rolling is divided into two stages: rough rolling and finish rolling. In order to obtain fine primary austenite grain size, the cast slab is rolled with a large reduction during the rough rolling stage, and the reduction rate of each pass is controlled to be above 15% under the conditions allowed by the rolling mill load, and/or each pass The pass reduction is more than 25mm. In order to obtain fine grain size and good plate shape, the total pass deformation ratio in the rough rolling stage can be controlled to be greater than 80%, and/or the reduction rate in the last pass of finishing rolling can be no less than 16%. In this article, "deformation ratio" refers to the ratio (percentage) of the reduction of the thickness of the steel plate after rolling to the initial thickness: that is, deformation ratio = (initial thickness - existing thickness) / initial thickness * 100%.

Since the present invention involves off-line heat treatment of steel types after rolling, there is no special requirement for the rolling temperature of the cast slab. However, in order to reduce the rolling load, the finishing rolling and coiling temperatures are as high as possible. From the continuous transformation curve in Figure 1, the ɑ→γ transformation point of the steel type is about 780°C, so it is recommended to use a finishing rolling temperature above 880°C to ensure complete austenite zone rolling and achieve low The rolling load and the stability of the rolling load are conducive to obtaining high-quality plate shape in the future; when the steel plate is thicker, the finishing rolling and final rolling temperature can be appropriately lowered, but it shall not be lower than 850°C. After rolling, the steel coil is cooled by laminar flow cooling to a temperature between 550-680°C for coiling. If the cooling rate is too high, the cooling rate will be too low, which will lead to coarse grains in the steel coil, which is unfavorable to the coiler; if the temperature is too low, it is easy to form Bainite. structure, improve the strength of the steel plate, and increase subsequent development Rolling and straightening difficulty.

The steel coils that have been cooled to room temperature are uncoiled and straightened before being cut into plates. The steel plates are quenched and tempered to obtain high strength and hardness and ensure wear resistance.

The quenching temperature directly affects the grain size of the subsequent martensite structure, which in turn affects the toughness of the steel plate. In order to ensure that the matrix is fully austenitized, a heating temperature of 30-50°C above the Ac3 point is generally used. If the heating temperature is too high, it is easy to coarsen the austenite grains. After quenching, the martensite structure will become coarse and the toughness will deteriorate. On the other hand, if the heating temperature is too low, the austenitization will be insufficient, and the complete martensite structure will not be obtained after quenching. Resilience is bad. The holding time also has a similar rule to the quenching performance. If the time is too long, it will easily make the grains coarse, increase energy consumption, and increase the cost. If the time is too short, the austenitization will be insufficient, and the hardness and strength after quenching will not meet the requirements. In order to obtain outstanding low-temperature toughness, the present invention specifically adopts a critical zone quenching process to quench the steel plate. There is undissolved acicular ferrite in the quenching structure in the critical zone. Although these undissolved acicular ferrite will reduce the strength, under the action of external force, it reaches the strength limit before martensite, causing cracks to first occur. It sprouts, expands, and absorbs energy, thereby improving toughness. Therefore, it is required to control the quenching temperature between -5℃ and +20℃ above the Ac3 point, that is, between 820-845℃, so as to obtain better low-temperature toughness. The quenching holding time T1 is calculated from the center of the steel plate to the temperature, and is 1.5-2 times (min) of the plate thickness H (mm). After the steel plate comes out of the furnace, it is directly water-quenched to room temperature with a cooling rate of ≥50°C/s.

Tempering treatment mainly slows down and eliminates quenching stress and improves plasticity and toughness. Higher tempering temperatures can easily reduce the strength and hardness of steel plates too much, making them unable to meet design requirements and increasing costs. Therefore, the tempering process parameters of steel plates should be limited. In the present invention, the steel plate is tempered in the range of 200-240°C. The tempering and heat preservation time T2 starts from the center of the steel plate to the temperature. The time is 2-3 times (min) of the plate thickness H (mm), but the minimum is not allowed. Less than 12 minutes. Finally, the quenched and tempered steel plates are subjected to finishing treatment (straightening, trimming), and are released from the factory after passing the performance requirements.

In an exemplary embodiment, the following process path is adopted: deep S removal from molten iron (to ensure low S content in steel) → top and bottom combined blowing of converter (to control C content) → refining outside the furnace → continuous casting (machine cleaning) → plate Billet reheating → controlled rolling → controlled cooling → coiling → uncoiling → straightening → plate cutting → heat treatment (quenching + tempering) → finishing → delivery.

The process of the present invention can be used to produce high-hardness, corrosion-resistant steel plates with a thickness of 8-20 mm. The yield strength of the steel plate is above 1100MPa, the tensile strength is above 1300MPa, the elongation is ≥12%, the hardness is 450±30HBW, and the -40℃ impact energy is ≥60J. Combined with the corrosion-resistant design of the steel type, the steel plate has good wear resistance and delayed cracking resistance. In a large-particle, high-density seawater slurry transportation environment, the abrasion resistance can be more than twice that of ordinary Q235B pipes.

The invention has the following advantages:

The present invention adopts a simple and economical C-Mn composition design, supplemented by a small amount of Nb and Ti micro-alloying elements, to achieve high hardness of the steel; it uses corrosion-resistant elements such as Cu, Ni, Cr, etc. to increase the matrix potential and inhibit corrosion. occur, Improved corrosion resistance of steel plates. As a result, the steel type has good corrosion resistance in corrosive and wear environments, especially when transporting large particles and high-density seawater slurry, the corrosion resistance is more than twice that of ordinary pipes.

The invention relates to a steel type that has good low-temperature impact toughness and cold-bending performance, meets the pipe-making processing requirements of subsequent dredging pipelines, and can realize easy pipe-making of high-hardness steel plates on the basis of existing equipment.

The steel type involved in the invention has excellent low-temperature toughness and corrosion resistance, significantly improves the delayed cracking resistance of the steel plate, reduces the risk of cracking and leakage of the dredging pipe during service, improves dredging efficiency and reduces maintenance costs.

The invention relates to a steel type that has a simple production process and low content of precious alloy elements, reduces production difficulty and production cost, and is conducive to the wide-scale promotion of the steel type.

Aiming at the service conditions of dredging pipelines, the present invention provides a high-hardness corrosion-resistant steel plate. The steel plate forms a high-hardness martensitic structure after heat treatment, with a yield strength of ≥1100MPa, a tensile strength of ≥1300MPa, and an elongation of ≥ 12%, hardness is 450±30HBW, -40℃ impact energy ≥60J. It has excellent wear resistance and improved corrosion resistance. The wear resistance is twice that of existing ordinary carbon steel materials. It also has good delayed cracking resistance and is easy to weld and cold-bend. The high-strength dredging pipe made of it is especially suitable for the field of large particle and high-density slurry transportation. It is not prone to cracking and leakage during use, which is not available in other currently known patented steel types.

Compared with the prior art, the steel type involved in the present invention has significant differences in composition and performance from the comparative patent:

In terms of ingredients, Comparative Patent Publication 1 (CN102776445A) requires the addition of 0.01-1.0% Mo, Ca and REM, and also requires an N content of 0.01-0.1%. The strength is improved through N, and the upper limit of the Mn content reaches 5%, which is close to mid-range. Manganese steel composition.

The contents of C, Mn, and Cr in the ingredients of Comparative Patent Publication 2 (CN101886225A) are as high as 0.4-0.9%, 14-16%, and 5-10% respectively, and require the addition of various rare elements such as Pr, Dy, Gd, and Nd.

Comparative Patent Publication 3 (CN10893001A) has lower Cr in the composition, but higher Al content, which is detrimental to toughness. The steel type of the present invention improves the corrosion resistance through Si, Cr, Cu, and Ni, and the contents of these elements are different from the comparative patent disclosure 3.

In addition, the performance requirements of the steel of the present invention are also different from those of Comparative Patent Publications 1-3.

The steel of the present invention requires a yield strength of more than 1100MPa, an elongation of ≥12%, a -40°C low-temperature impact energy of ≥60J, and clearly has good delayed cracking resistance, which is not available in steel types 1-3 of the comparative patent disclosures. Among them, the yield strength range of Comparative Patent Publication 1 is relatively wide, from 300MPa to 2500MPa. Although high strength can be achieved, plasticity is sacrificed and the elongation cannot be guaranteed, which limits the scope of application; Comparative Patent Publication 2 has a high content of Although the strengthening element can achieve a hardness exceeding 50HRC, the cost is too high and the elongation cannot be guaranteed, which affects the processing performance; and the comparative patent disclosure 1 and 2 steel types do not have good low-temperature punching properties. Hit toughness.

Description of the drawings

Figure 1 is the CCT curve of the steel according to the present invention.

Detailed ways

The present invention will be further described below with reference to examples, but the present invention is not limited to these examples.

Example 1-22

1) Smelting and casting

The molten steel is smelted in a 500kg vacuum induction furnace and cast into 100kg steel ingots.

2) Heating of cast slab

The heating temperature of the cast slab is above 1230°C, the total heating time in the heating furnace is not less than 2 hours, and the soaking time is not less than 40 minutes.

3) Rough rolling and finish rolling

In the rough rolling stage, the reduction rate of each pass is more than 15%, and/or the reduction amount of each pass is more than 25mm, and/or the total deformation ratio of the pass is greater than 80%.

In the finishing rolling stage, the reduction rate of the last pass is not less than 16%, and the finishing rolling temperature is ≥880°C.

4) Cooling and coiling

Cool to 550-680℃ and then coil.

5)Quenching and tempering

The quenching heating temperature is 820-845°C, and the quenching holding time T1 is 1.5H-2H, unit: min, where H represents the plate thickness in mm; after the steel plate is released, the water is quenched to room temperature, and the cooling rate is ≥50°C/s;

The tempering temperature is 200-240℃, the tempering holding time T2 is 2H-3H, unit: min, where H represents the plate thickness in mm, and T2≥12min.

6) Finishing treatment

Includes straightening and trimming.

Comparative Example 1-4

Comparative Examples 1-4 were manufactured using a method similar to that of the Examples. However, one or more of the elemental compositions and manufacturing processes of Comparative Examples 1-4 do not fall within the scope of the present invention.

Table 1 shows the composition of the steel plates of the Examples and Comparative Examples, and Table 2 shows some of the processes of the Examples and Comparative Examples. Process parameters, Table 1 shows the performance parameters of the embodiments and comparative examples.

The cracking time was measured as follows: A U-bend immersion test was used to evaluate the delayed cracking resistance of steel plates. Bend a 2*20*90mm sample into a U-shape with a radius of 10mm, use a clamp to load the sample until both sides of the sample are parallel, then soak it in 0.1mol/L hydrochloric acid, and replace the solution every 24 hours. During the test, observations were made twice a day, the specific cracking time of the sample was confirmed based on video playback, and the cracking time of the sample was recorded. The shorter the cracking time of the sample, the worse the delayed cracking resistance, and the higher the risk of delayed cracking under corrosion conditions. It is generally believed that no cracking for more than 300 hours indicates good delayed cracking resistance.

As can be seen from Table 3, the hardness of the steel plates involved in the present invention all reaches the 450HBW level, and the tensile properties also meet the design requirements, thus having excellent abrasion resistance (the abrasion resistance of the steel plates of the present invention is more than twice that of ordinary Q235B steel plates) . In particular, the delayed cracking time of the steel plate of this application is above 600 hours, which reflects that the steel plate of this application has excellent delayed cracking resistance.

The present invention is compared with the current conventional 450HBW grade wear-resistant steel as a comparative example.

Comparative Examples 1-4 are designed with a C-Si-Mn composition, in which the Mn content is about 1.6%, the Cr content is 0.4-1.2%, and no Cu and Ni are added. Comparative Example 1 adopts a finish rolling temperature of 820°C, but the impact energy at -40°C is only 33J. In the U-shaped bending immersion test, cracking occurred within 48 hours, and its low-temperature toughness and delayed cracking resistance are far lower than those of the steel of the invention. kind; Comparative Example 2-4 adopts a finishing rolling temperature of 880-900°C, and its low-temperature impact energy of -40°C is 23-33J. The maximum cracking time in the U-shaped bending immersion test is only 57h, which is much lower than that of the present invention. Steel type. Therefore, the comparative example does not have the delayed cracking resistance required by dredging conditions and is not suitable for the production of dredging pipelines.

The abrasion-resistant steel plate involved in the present invention can be used for the production of slurry dredging pipes, and is widely used in the fields of sea reclamation, waterway dredging, inland river dredging and slurry transportation, replacing the current Q235 and Q345 level ordinary dredging pipelines, thereby improving production efficiency and reduce operating costs.

table 3

Note: *For Example 1, Example 3 and Comparative Example 1, -40°C AKV in parentheses is the correction value. The thickness of the steel plates in Example 1, Example 3 and Comparative Example 1 is 8mm, and the size of the impact specimen is 7.5*10*55mm, while the size of the full-size impact specimen is 10*10*55mm. Therefore, the correction values of Example 1, Example 3 and Comparative Example 1 after conversion into full-size impact specimens are respectively: 49*(10/7.5)=65J, 52*(10/7.5)=69J and 33*( 10/7.5)=44J.

Claims

A steel plate containing the following chemical elements in wt%: C: 0.17-0.22%, Si: 0.1-0.3%, Mn: 1.0-1.4%, P≤0.015%, S≤0.005%, Al: or Two types, the balance is Fe and unavoidable impurities;

The elements N, Nb and Ti satisfy the following inequality: 5.68N≤Nb+Ti≤0.044;

The elements Cu and Ni satisfy the following inequality: Cu/Ni≤2.0.
The steel plate according to claim 1, wherein the steel plate further contains Cr≤2.0%, W: 0.01-0.5%, Mo: 0.01-0.5%, Sb: 0.01-0.2%, REM: 0.01-0.2%, V : 0.01-0.2% and Ca: 0.001-0.01% or more.
The steel plate according to claim 1 or 2, wherein the steel plate meets more than one of the following:

Cu: 0.29-0.60%;
6.65N≤Nb+Ti≤0.04;
0.7≤Cu/Ni≤2.0.
The steel plate according to claim 1 or 2, wherein the thickness of the steel plate is 8-20 mm.
The steel plate according to claim 1 or 2, wherein the steel plate meets more than one of the following properties: yield strength ≥ 1100MPa, tensile strength ≥ 1300MPa, elongation ≥ 12%, hardness 450±30HBW, -40 ℃ impact energy ≥60J, cracking time is more than 600h.
The steel plate according to claim 5, wherein the corrosion resistance of the steel plate is more than twice that of ordinary Q235B steel plate.
The method for manufacturing the steel plate according to any one of claims 1-6, wherein the method includes the following steps:

1) Smelting and casting

Smelting and casting molten steel to obtain a cast slab;

2) Heating of cast slab

The heating temperature is above 1230°C, and the total heating time in the heating furnace is not less than 2 hours, of which the soaking time in the soaking section is not less than 40 minutes;

3) Rough rolling and finish rolling

In the rough rolling stage, the reduction rate of each pass is more than 15%, and/or the reduction amount of each pass is more than 25mm, and/or the total pass deformation ratio is greater than 80%;

In the finishing rolling stage, the reduction rate of the last pass is not less than 16%, and the finishing rolling temperature is ≥880°C, preferably 880-898℃;

4) Cooling and coiling

Cool to 550-680℃, then coil;

5)Quenching and tempering

The quenching temperature is 820-845°C, and the quenching holding time T1 is 1.5H-2H, unit: min, where H represents the plate thickness in mm; after the steel plate is released, the water is quenched to room temperature, and the cooling rate is ≥50°C/s;

The tempering temperature is 200-240℃, the tempering holding time T2 is 2H-3H, unit: min, where H represents the plate thickness in mm, and T2≥12min;

6) Finishing treatment

Straightening and trimming.
The method according to claim 7, wherein in step 4), cooling to 560-680°C, and then coiling.
The method according to claim 7 or 8, wherein in step 5), the quenching temperature is 828-845°C; and/or the tempering temperature is 210-240°C, preferably 220-240°C.