The development history of heavy plate production in China was sorted out, and the development of heavy plate in China was divided into five stages, i. e, initial stage, accumulation stage, development stage, maturity stage and optimization stage, The characteristics of each stage were analyzed from the aspects of mill specification, equipment level, capacity scale and so on; The technical progress and development of key processes and equipment for heavy plate, such as hot delivery and hot charging, reheating furnace, rolling mill and leveler were described; The development, application and advancement of typical heavy plate products, such as special shipbuilding steel and offshore engineering steel were elucidated; The future development of heavy plate in China was prospected and suggestions were put forward.
Nickel (Ni) is an important alloying element widely used in steel, which not only significantly enhances the corrosion resistance and heat resistance of stainless steel, but also improves the strength, ductility, and toughness of alloy steel. Nickel-based alloys are also applied in the manufacturing of high-temperature aerospace structural components due to their high strength and good heat resistance. Although nickel has been fully utilized in steel and alloys, especially in improving the toughness/cryogenic toughness of steels, the physical metallurgical mechanism of nickel in enhancing the toughness of steel has not been fully understood. In this paper, the application and research progress of nickel in steel and alloys are first briefly reviewed, and then the mechanism study of nickel in improving the impact toughness and welding toughness of steel are summarized. It is emphasized that the improvement of impact toughness of steel by nickel is mainly achieved by grain refinement, dislocation movement promotion, increment of high-angle grain boundaries, and stabilizing austenite, while the welding toughness is enhanced by effectively increasing the content of acicular ferrite and reducing the harmful phase of martensite-austenite islands (M/A). As a new approach to strengthening and toughening steel, the research status of Ni-Cu alloying in low-alloy high-strength steel are briefly described. Finally, the development trend of Ni-containing steel is provided.
To solve the problem of thick oxide scale and color variation between edges and center of S355GD-M strip, in-situ observations were conducted on oxide scale formation during the heating, and experimental studies were performed on the structural transformation of surface oxide scale after strip cooling at different coiling temperatures. Results indicate that oxide scale on S355GD-M slab surfaces during heating progresses through several stages: intergranular growth, dense growth, loose growth, aggregated longitudinal growth, and stable growth. At the beginning of oxidation, the specimen surface is not covered by oxidation products, allowing Fe atoms in the substrate to fully contact O2. Due to higher energy at grain boundaries within the substrate, oxidation products nucleate preferentially at grain boundaries. As temperature further increases, grain boundaries become fully covered by oxidation products, and the oxidation reaction proceeds toward non-grain boundary regions until the specimen surface is completely covered by dense oxidation products. At this time, the outermost layer of oxidation products in contact with O2 exhibits the highest oxygen concentration, where FeO is oxidized to Fe2O3 and Fe3O4 and begins longitudinal growth. As the coiling temperature decreases, the proportion of eutectoid structure first increases and then gradually decreases. Conversely, as the cooling rate decreases, the proportion of eutectoid structure gradually increases. Based on experimental results, a CCT curve for the FeO structural transformation within the iron oxide scale was plotted, revealing compliance with the C-curve pattern, with a nose temperature around 500 ℃. Based on in-situ observation results, higher temperatures accelerate iron oxide scale growth. An effective strategy to reduce iron oxide scale thickness is to minimize the strip's prolonged exposure to high temperatures. By lowering the starting rolling temperature, increasing the finishing rolling temperature, and raising the strip rolling speed, the high-temperature dwell time was effectively reduced, achieving the goal of thinning the oxide scale. The structural differences in oxide scale primarily result from varying cooling rates across different strip locations. Based on experimental results of FeO eutectic phase transformation, the coiling system was adjusted by setting the coiling temperature near the nose tip temperature, thereby reducing structural variations between the strip edges and center. Following process optimization, the thickness of the oxide scale on the surface of S355GD-M strip reached about 12 μm. Both the edge and center regions exhibited an oxide scale structure composed of Fe3O4 + FeO + eutectic phase. This significantly improved surface color defects, enhancing the surface quality of the hot-rolled product.
Currently, the production of 690 MPa grade marine platform steel plates usually adopts offline heat treatment, which not only wastes a large amount of heat but also takes a long time. This paper improves the existing heat treatment process and adopts the full-process online heat treatment (i.e., online quenching + online tempering) process. The microstructure of the plates after online heat treatment is observed by OM, SEM, TEM, and EBSD, and the yield strength, tensile strength, and impact energy are tested by a universal testing machine to explore the laws of microstructure evolution and property changes of the steel plates under different online quenching temperatures. The results show that the microstructure of the experimental plates after heat treatment is mainly composed of tempered martensite, ferrite, and carbides. With the increase of quenching temperature, the amount of martensite and the width of laths in the microstructure increase, the amount of carbide precipitation increases, the proportion of high-angle grain boundaries decreases, the yield strength and tensile strength increase, and the impact energy decreases. After online quenching at 850 ℃, although the yield strength of the experimental plates is lower than that at 900 ℃, the yield-to-tensile ratio and impact toughness are higher than those at 900 ℃, and the comprehensive mechanical properties are the best. The research results provide theoery support for the technology optimization of E690 plate.
With the rapid development of deep-sea oil and gas engineering and other fields, the requirements for material properties are getting higher and higher. In order to improve the application performance of S32707 hyper duplex stainless steel in deep-sea oil and gas engineering, this paper studies the effect of solution temperature on its mechanical properties and corrosion resistance. According to the calculation results of Themo-Calc thermodynamic simulation software, the S32707 samples after rolling were solution treated at different temperatures (1 055 ℃, 1 095 ℃) for 1 h. The microstructure of S32707 hyper duplex stainless steel at different solution temperatures was characterized by metallographic, scanning electron microscopy, electron probe and backscattered electron diffraction. The mechanical properties and corrosion resistance of S32707 samples were measured by tensile testing machine, impact testing machine, material surface performance comprehensive tester and electrochemical comprehensive tester. When the solution temperature is above 1 055 ℃, there are only ferrite and austenite in the S32707 sample. With the increase of solution temperature, the concent of α/γ ratio increases, the content of Cr, Mo and Si in the ferrite phase decreases, the content of Ni increases, and the austenite is the opposite, that is, the content of alloying elements between the two phases gradually tends to be consistent. At higher solution temperature, the increase of ferrite phase content and the transfer of alloying elements improve the strength, hardness, and wear resistance of the samples, and reduce its plasticity, toughness and elongation. At lower solution temperature, the impedance arc of S32707 sample is larger, the anodic dissolution rate is lower, and the passivation zone is wider, indicating that its corrosion resistance is better.The research results provide data support for the production of hyper duplex stainless steel and its application in deep-sea oil and gas engineering,seawater desalination and other fields.
Annealing is typically employed to mitigate residual stresses in materials. However, for low-carbon micro-alloyed high-strength steels, their low carbon content results in insufficient precipitation-induced plasticity strain to adequately relieve residual stresses. This study investigates the influence of plasticity behavior on residual stress at different annealing stages through stress measurement, microstructure characterization, and partitioning of plasticity strain. The results indicate that the partitioning plasticity induced by Mn partitioning behavior is greater than the precipitation plasticity caused by the precipitation of alloy carbides, making it the primary mechanism for residual stress relaxation in low-carbon steel. The partitioning plasticity strain parameter K associated with Mn partitioning is 2.691×10-5 MPa-1, while the precipitation plastic strain parameter K for alloy carbide precipitation is 1.303×10-5 MPa-1. After holding at the Mn partitioning stage for 15 minutes, the absolute residual stress of the tested steel decreases from 980 MPa to 330 MPa, a reduction of 66.3%. In contrast, after holding at the alloy carbide precipitation stage for 15 minutes, the absolute residual stress decreases from 323 MPa to 277 MPa, a reduction of only 14.2%. Therefore, partitioning plasticity is the dominant mechanism for residual stress relaxation in low-carbon steel.
In order to develop an ultra-high strength automobile beam steel with a tensile strength of 850 MPa to ensure traffic safety and achieve vehicle lightweighting, a new experimental steel with a novel chemical composition was designed. Phase transformation behaviors of the experimental steel under continuous cooling and isothermal conditions were investigated by using dynamic phase transformation instrument experiments. The effects of coiling temperature on the microstructure and properties of hot-rolled plates were studied by using hot rolling experiments at laboratory. The results of the dynamic phase transformation instrument experiments showed that bainitic microstructures can be obtained at relatively low cooling rates for experimental steel. Under the condition of isothermal holding for 800 s, as the isothermal temperature decreased, the hardness of the experimental steel first increased and then decreased, reaching its maximum hardness at an isothermal temperature of 550 ℃. The results of the hot rolling experiments indicated that at a coiling temperature of 590 ℃, the mechanical properties of the plate met the standard requirements for automobile beam steel with a tensile strength of 850 MPa, with the main strengthening mechanisms being phase transformation strengthening, fine-grain strengthening, and precipitation strengthening.The research prodives the theotry support for the high-strength automobile beam production.
In response to the issue of significant differences in mechanical properties among different thickness gauges of Nb-Ti micro-alloyed steel produced by the short process line of Shougang Jingtang, this paper comparatively analyzed the hot rolling processes of different thickness gauge strips, and observed the microstructure morphologies and second phase precipitates of strips by means of OM, SEM, EBSD and TEM, and quantitatively calculated the contribution of various strengthening mechanisms to the strength. The results show that although the same finishing rolling temperature and coiling temperature were designed for the production of different thicknessgauge strips, the finishing rolling temperature would vary due to the different rolling speeds under the endless rolling mode. The thicker the strip, the lower the finishing rolling temperature. Under the same coiling temperature control, the finishing rolling temperature of thick gauge strips is lower, which leads to less cooling water in the early stage of laminar cooling, resulting in weakened fine grain strengthening and precipitation strengthening effects, and ultimately lower mechanical properties of thick gauge strips. The microstructure of strips of different gauge strip is mainly composed of ferrite and a small amount of pearlite. The average ferrite grain sizes of 2.0, 1.8, 1.6, 1.5 mm gauge steel strips are 6.83, 6.07, 5.23, 4.62 μm respectively, and the fine grain strengthening contributions are 211.8, 224.6, 242.0, 257.5 MPa respectively, accounting for about 53% to 54% of the total strength. The second phase precipitates in Nb-Ti microalloyed steel mainly include TiN, Ti4C2S2, Nb(CN), TiC and (Nb, Ti)(C, N) composite precipitates. Due to the low content of Nb and Ti elements in the experimental steel and the influence of controlled rolling and controlled cooling process, the precipitation strengthening contributions are relatively small, being 21.6, 34.5, 49.2, 63.5 MPa respectively, accounting for about 5% to 14% of the total strength; the solid solution strengthening contribution is 68 MPa, accounting for about 14% to 18% of the total strength. The difference in yield strength between 2.0 mm and 1.5 mm gauge steel strips mainly comes from the difference in fine grain strengthening and precipitation strengthening, which are 45.7 MPa and 41.9 MPa respectively.
Aiming at the problems in the current production of SWRH82B wire rods, including poor overall performance along the entire length, abnormal structures (such as high-level network cementite and martensite), low and uneven sorbitization rate, this paper conducted a finite element simulation calculation of the temperature field throughout the entire rolling process of SWRH82B wire rods using DEFORM-3D software, based on a high-speed wire rod production line of a steel plant. The influence of plastic deformation of the workpiece on heat exchange between its surface and core during rolling was analyzed. Meanwhile, the optimization of water cooling processes was investigated.The results show that: In the roughing stage, the surface temperature of the workpiece shows a gradual decreasing trend, while the core temperature decreases slowly; in the intermediate rolling stage, the surface temperature rises, and the core temperature decreases more significantly. During the pre-finishing stage, the cross-sectional area of the workpiece gradually decreases, and the entire section heats up due to heat generation from plastic deformation, leading to increased temperatures in both the surface and core. In the finishing and reducing-sizing stages, with higher rolling speeds, the surface and core temperatures of the workpiece briefly increase, then rapidly decrease after water cooling, followed by a rise in surface temperature due to core reheating. When the convective heat transfer coefficients of No.1 to No.4 water cooling boxes and No.5 water cooling box are 400 W/(m2·℃) and 300 W/(m2·℃), respectively, the average surface temperature of the wire rod at the laying head is approximately 919 ℃, and the core temperature is about 939 ℃.After optimizing the water cooling process, the cross-sectional temperature of the workpiece becomes uniform, with the simulated temperatures basically agreeing with the measured ones (error within ±10 ℃). Meanwhile, the martensitic structure in the core of the wire rod is significantly reduced. Compared to before optimization, the tensile strength of the wire rod increases by approximately 40 MPa, and the reduction of area increases by about 8%.
The air preheater of heating furnace is an important energy-saving device in the steel rolling heating furnace production process. At present, the widely used circular-tube cross-flow air preheater suffers from large volume, low heat transfer efficiency, and high exhaust gas temperature. The twisted elliptical tube variable-space counter-flow air preheater, which uses twisted elliptical tubes as heat exchange elements, can improve the utilization rate of waste heat from flue gas and reduce fuel consumption. In this paper, using FLUENT software with air and flue gas as working fluids, a numerical simulation of twisted elliptical tubes and circular tubes was conducted, and their heat transfer efficiencies were compared and analyzed. The results indicate that, compared with circular tubes, the fluid flow in twisted elliptical tubes is more turbulent and the heat transfer performance is better. Using the heat balance method, resistance characteristic method, and thermal resistance analysis method, a relevant prediction model was derived, and factors affecting the comprehensive heat transfer performance and thermal efficiency of the air preheater were analyzed. Compared with the circular-tube cross-flow air preheater, the twisted elliptical tube air preheater can significantly enhance heat recovery and increase the overall heat utilization efficiency of the heating furnace by more than 6%.
In response to the severe uneven wear of roll during the production of silicon steel using conventional parabolic roll shapes on a certain hot-rolled coil production line, local high points were prone to occur on the rolled plate for the later stage of rolling, which can cause ridge buckle for the latter process. And crown was controlled with difficulty. The article developed a set of composite roll profile curves for work rolls, which combined parabolic roll profile and double taper roll profile to improve uneven wear of rolls and enhance the quality of rolled section. The numerical simulation technique was used to establish an elastic-plastic finite element model for the composite profile of hot rolling work rolls. The simulation results showed that compared with conventional profiles, the profile have greater advantages in the efficiency of bending force control and the improvement of hot rolling profile and edge drop control level. The roll type was applied to the production site for debugging and verification. The results showed that after the new roll type was put into use, the problem of uneven wear of the roll was significantly improved. The number of silicon steel rolling blocks per unit roll period increased from 40 to 50, and the compliance rate of hot rolling profile C40 ≤ 30 μm increased from 46.2% to over 90%.
20-high precision rolling mill can make full use of the strong rolling ability of small diameter work roll and the high rigidity of the tower roll system to achieve high-yield and high-quality production of high strength steel. However, affected by the non-uniform contact stress in actual production, backing bearings of 20-high precision rolling mill are prone to non-uniform wear and fatigue failure. Therefore, starting with the internal relationship between contact stress and service ability of backing bearings, a simulation and analysis model of roll system and rolling piece coupling of 20-high rolling mill was established to analyze the effect of rolling parameters and flatness control parameters on the contact stress between backing bearing roll and second intermediate roll. The results show that the backing bearing roll at the rolling inlet side is the backing bearing which is easy to produce the maximum contact stress, and the stress peak can be changed under the influence of parameters. Increasing the rolling force will cause an overall increase in the contact stress between the rolls, but will not alter its distribution pattern. Increasing the amount of roll shifting will cause the indirect contact stress between the rolls to shift towards the direction of roll shifting, resulting in a slight increase in the peak contact stress between the backing bearing roll and the second intermediate transmission roll. Changing the AS-U regulation will force stress peaks in the inclined direction of the backing bearing during the rolling process under high pressure by tilting the core axis. Based on the influence of rolling force, roll shifting value and AS-U regulation on the contact stress between backing bearing roll and second intermediate drive roll, a kind of long service VCR roll profile of the backing bearing for 20-high precision rolling mill is proposed. Through analysis and verification, the roll profile can effectively eliminate the stress peak at the end of the backing bearing, and the peak of the contact stress at the two sides of the backing bearing in the middle position can be reduced by 5.83% on the original basis, which has the effect of extending the life of backing bearing roll under the condition of long cycle service.
During strip rolling, the thickness of the strip can be fine-tuned by adjusting the tension, and the rolling force can be reduced by increasing the strip tension. Therefore, maintaining a high tension of the strip and the rapid response of tension to changes in strip thickness are effective means to ensure the stable rolling of silicon steel and improve the rolling quality of silicon steel. To realize the high-tension control of the silicon steel cold tandem rolling mill, this paper proposes a measure of adopting a tension roll set at the exit of the cold tandem rolling mill. The equipment functions of the tension roll unit are discussed, and the key parameters are calculated and explained. Practice shows that during the production of silicon steel, the application of the tension roll set at the exit of the cold tandem rolling mill not only solves the problem of the high tension required at the mill exit, but also solves the problem of strip tension loss at the exit of the last stand mill at the moment of shearing. The thickness fluctuation of the strip head is within ±3 μm, the length of unqualified thickness is 0 m, and the rolling yield of silicon steel is improved.
Traditional strip shape recognition methods often struggle to simultaneously address both local and overall features when dealing with the complex strip shapes of wide and thin gauge strips. To enhance the accuracy of strip shape defect recognition algorithms for complex strip shapes, a computational model is proposed that targets both local and overall strip shape recognition. This model utilizes a neighboring channel smoothing algorithm to extract local strip shape features, resulting in more accurate overall strip shape values. Additionally, an improved cosine similarity-optimized fuzzy classification algorithm is employed to recognize overall strip shape patterns from the strip shape curves. By integrating the extracted local features with the remaining strip shape values, the actual local strip shape feature values are obtained. Field application results indicate that, compared to traditional Euclidean distance and Manhattan distance formulas, the improved cosine similarity-optimized fuzzy classification algorithm based on local wave shape extraction better measures the similarity between vectors in high-dimensional space, thereby improving recognition accuracy.
In order to solve the problem that the current rolling force prediction model can not accurately reflect the actual rolling force, a new rolling force prediction model based on RBF neural network was established by revising the Sims model twice. Based on the actual production data of Q345 steel, the data set required for RBF neural network prediction is formed through the reverse calculation of the actual deformation resistance and the data normalization processing. Then the network structure is optimized to achieve the accurate prediction of deformation resistance, and the theoretical rolling force is obtained by using the modified deformation resistance calculation. In order to further improve the rolling force accuracy, starting from the Sims model structure, RBF neural network was used to modify the model proportion coefficient (the ratio of actual rolling force to theoretical rolling force), and the rolling force correction coefficient was obtained. According to the performance evaluation, the maximum error of the rolling force after first correction is 21.595%, and the maximum error of the rolling force after second correction is only 3.631%. This paper provides a new idea for improving the precision of rolling force.
For an extended period, pickling technology has been widely employed for surface descaling of hot-rolled plates, coils, and other steel products. However, the corrosive nature of acids not only compromises the surface quality and performance of treated products but also causes damage to operators and equipment, while polluting the production environment. Against the backdrop of increasingly stringent national environmental protection policies, enterprises are now mandated to achieve zero pollutant emissions. In response, domestic and international research institutions have intensified their efforts in investigating descaling process technologies and equipment, leading to the development of more environmentally friendly novel techniques and processes. This paper conducts a comprehensive comparative analysis of the current applications of acid-free descaling technologies and proposes that the future development direction of hot-rolled product descaling technology lies in low-cost, acid-free solutions.
The development of carbon capture, utilization and storage (CCUS) technology has effectively promoted the recycling and utilization of carbon dioxide. To meet the manufacturing requirements for pressure vessel storage tanks used in transport ships for storing liquefied carbon dioxide, the steel industry has developed a new ultra-high strength plate, i.e.P690QL2 plate. This paper investigates the lamination phenomenon observed in the tensile fracture of a 30 mm thickness P690QL2 plate produced by a domestic manufacturer. Advanced equipment such as optical microscopy (OM), scanning electron microscopy (SEM), and ultrasonic testing (UT) were employed to analyze the causes of the lamination in the tensile fracture of the plate. The results indicate that after the tensile test of the 30 mmthickness plate, no significant defects were found via ultrasonic testing and dye penetrant inspection. The hardness difference between different locations on the cross-section near the tensile fracture was approximately 14HV. Observations under OM and SEM revealed that the steel microstructure contained grade 0.5 type D inclusions, primarily TiN non-metallic inclusions. Energy Dispersive Spectroscopy (EDS) analysis results showed that the mass fractions of Ni element at the crack and the matrix were 2.6% and 1.7% respectively, indicating the presence of Ni element segregation in the plate. Concurrent with Ni segregation, microstructural segregation also occurred, resulting in a segregation zone composed mainly of higher strength and hardness tempered martensite. This led to multi-directional stresses of plate during the tensile process, all exceeding the yield strength, which is identified as the cause of the lamination in the tensile fracture. Therefore, in actual production, adopting a continuous casting process with low superheat molten steel and controlling an appropriate casting speed is recommended. Simultaneously, increasing the slab heating temperature to 1 250 ℃ and prolonging the holding time are beneficial for dissolving TiN inclusions, which can prevent the occurrence of the lamination phenomenon in the tensile fracture of P690QL2 plates.
In response to the insufficient research and application of Q460 and above grade high-strength steel for construction in China, a chemical composition design based on low-carbon high-strength steel and adding appropriate amounts of Nb, Ti, Cr, and B alloy elements was adopted. The Q550GJE high-strength steel plate for construction was trial produced using TMCP process and reasonable heat treatment process. The effect of different tempering temperatures on the mechanical properties and microstructure of Q550GJE plate was studied. The results showed that with the increase of tempering temperature, the yield strength of the test plate first increased and then gradually decreased, the tensile strength gradually decreased, the yield strength ratio gradually increased, the elongation first increased and then decreased, and the impact energy first increased and then decreased; After tempering, the microstructure of the test plate is mainly composed of ferrite and granular bainite, with island shaped M-A structure dispersed in the ferrite matrix. As the tempering temperature increases, the M-A structure gradually decomposes, the ferrite size gradually increases, and the granular bainite content gradually decreases. At 700 ℃ tempering, the ferrite size is the largest. Research has shown that when the tempering temperature is 550 ℃, the test plate has good strength and toughness as well as a lower yield strength ratio.
The hot delivery and hot charging technology of continuous casting slabs is an important method for energy conservation and emission reduction in the steel products production process. To solve the surface crack problem during the hot delivery and hot charging of S45C carbon structural steel slabs, this paper established a slab heating simulation model. The temperature field and stress field of continuous casting slabs during heating were calculated under different furnace entry temperatures and heating processes, and the formation law and characteristics of thermal stress were analyzed. The results show that hot charging can effectively reduce the peak stress of slabs during heating; however, the higher the hot charging temperature, the greater the tensile stress on the slab surface when entering the furnace, and the higher the crack risk. Reducing the heating rate of slabs in the first heating section can effectively avoid the generation of peak stress in the high-temperature zone of slabs. Heating S45C slab according to the following heating scheme—furnace entry temperature of 500—600 ℃, heating coefficient of 7.8 min/cm, heat recovery section temperature of 800 ℃, preheating section temperature of 1 100 ℃, first heating section temperature of 1 160 ℃, second heating section temperature of 1 230 ℃, and soaking section temperature of 1 200 ℃—is conducive to reducing the crack risk of S45C with hot delivery and hot charging process. After optimizing the heating process, the crack rate of S45C slab decreased by 11.8%.
Hot-rolled coils produced by the ESP production line exhibit yellowing and rusting after pickling, with significant variations in the degree of yellowing among coils subjected to different coiling temperatures. Following adjustments to the pickling process, the strip surface becomes corroded, forming brown or yellow "flowing rust". This paper therefore conducts an analysis and investigation into this issue. The results indicate that the yellowing phenomenon arises from the combined action of the condition of the iron oxide scale on the surface of hot-rolled strips and the pickling process, where the key influencing factors include the thickness of the surface iron oxide scale, pickling speed, and squeezing efficiency. Mechanistic analysis of yellowing reveals that residual hydrochloric acid on the strip surface post-pickling reacts with matrix iron to form ferrous chloride, which further oxidizes to generate iron hydroxide and hydroxyl oxides, thereby causing yellowing. Maintaining the wettability of the strip surface after pickling can prevent oxidation reactions between the strip's matrix iron and air, ultimately eliminating surface yellowing and enhancing the overall product quality.
Aiming at the lightweight requirements for the vehicle, this paper investigates the influencing factors of the anti-concave property of automotive sheets. By combining anti-denting experiments with finite element simulations, the effects of pre-strain, sheet thickness, part curvature on the anti-concave performance of automotive sheets were studied. The experimental results show that when the pre-strain is less than 15%, the anti-concave stiffness of the sheet increases with the increase of pre-strain. When the pre-strain is greater than 15%, the anti-concave stiffness of the sheet decreases with the increase of pre-strain, which is due to the thinning of the sheet thickness caused by excessive pre-strain. For sheets with thicknesses of 0.65 mm and 0.70 mm, when the pre-strain is less than 3%, there is not much difference in the anti-concave stiffness of the sheets and the indentation depth is greater than 2 mm. When the pre-strain is greater than 3%, the anti-concave stiffness of the 0.7 mm thick sheet is significantly greater than that of the 0.65 mm thick sheet, and the indentation depth of both decreases sharply. As the pre-strain further increases, the indentation depth does not change significantly. This is because with the increase of pre-strain, the sheet undergoes work hardening, and the strength is significantly improved. The thickness of the sheet is one of the important factors determining its indentation resistance. Bake hardening has little effect on the anti-concave stiffness of sheet, but the depth of dents is significantly reduced compared to before baking. This is due to the increase insheet strength caused by strain aging during the baking process. The finite element simulation further verified that the curvature of the part is also one of the key influencing factors on its anti-concave property. The anti-concave property of the part increases with the decrease of its curvature. However,a smaller the curvature is not always better. It should be controlled within a reasonable range.
By comparing the strength, original austenite grain size, and ferrite grain size of Q235B, Q355B, and NM450 hot-rolled strips produced through two different roughing processes in hot rolling production line (separate arrangement of two-stand mills with individual reversible rolling and compact arrangement of two-stand mills with reversible continuous rolling), it was found that the compact arrangement of two-stand mills with reversible continuous rolling achieves fine grain strengthening for thick-gauge strips by refining the original austenite grain size, thereby significantly improving strip strength. However, this fine grain strengthening effect is less pronounced for thin-gauge strips with thicknesses of 3.0 mm or below. This roughing process features short pass intervals, which not only reduce the growth of recrystallized austenite grains but also promote cumulative deformation in non-recrystallized austenite, refining the original austenite structure and consequently the transformed ferrite structure. The grain refinement effect of this roughing process is particularly effective for producing thick-gauge products containing alloying elements such as Al, Ti+Al, and Nb+Ti+Al.
Under the condition of using different modules (chamfer module, groove module) of the width reduction machine in combination with different types of slabs (chamfered slab, right-angle slab),the displacement fields and equivalent stress-strain fields of the slabs are different. This leads to differences in the occurrence rate of edge defects and their distance from the edge of the hot-rolled strip, resulting in varied edge trimming losses and consequently affecting enterprise economic benefits.A finite element model for the width reduction process of the width reduction machine was established using the DEFORM platform, and simulation calculations were performed to analyze the side pressure, displacement field, and equivalent stress-strain field of the slabs under different working conditions. This study thus investigates the matching degree between the two modules of the width reduction machine and the slab types. The results show that, compared with the right-angle slab, the chamfered slab exhibits smaller maximum side pressure, thinner "dog-bone" thickness, and more uniform stress distribution during width reduction, indicating a better match with the chamfer module. Additionally, compared with the groove module, the slab under the side pressure of the chamfer module demonstrates smaller maximum side pressure, thinner "dog-bone" thickness, and more uniform stress distribution, achieving a better matching effect with the slab. Meanwhile, production practice confirms that the combination of the chamfer module and the chamfered slab reduces the occurrence rate of edge defects in strip and improves its edge quality, thereby enhancing enterprise economic benefits.
In response to the issue of white snowflake pattern defects appearing on the surface of zinc-aluminum-magnesium (Zn-Al-Mg) coated sheets produced by a certain factory, which have a thickness ranging from 0.35 mm to 0.8 mm and double-sided coating thicknesses of 70 g/m2 to 100 g/m2, an analysis of the coating's microstructure and chemical composition was conducted. It was found that after air-knife blowing, the oxides of Zn, Al, and Mg elements in the coating exhibited an uneven distribution on the coating surface. Due to the significant difference in the volume expansion rate of Mg oxides compared to those of Zn and Al oxides, the oxide film in regions rich in Mg elements was loose and unable to prevent O2 from entering the interior of the coating. This increased the oxidation duration of the coating, resulting in localized thickening of the oxide film and the formation of microscopic undulations, which manifested as snowflake pattern defects. Experimental results indicate that the snowflake pattern defects can be eliminated under the condition of air-knife blowing at 430 ℃ with the coating exposed to oxygen. Both air blowing and nitrogen blowing with an increase in tA (the end point of air-knife blowing) to 430 ℃ can eliminate the snowflake pattern defects. The former method offers low energy consumption and simple operation but has a narrow applicable range in terms of product thickness and coating thickness specifications, beyond which new defects may be introduced. The latter method, although it increases energy consumption by raising the strip entry temperature and bath temperature to ensure that tA reaches above 430 ℃, effectively eliminates snowflake pattern defects without forming any new ones, meeting the product's usage requirements and representing the optimal solution for eliminating Zn-Al-Mg coating snowflake pattern defects.
