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.

Marine engineering equipment faces significant challenges in achieving a coordinated design that integrates lightweight and properties with high strength and toughness under harsh service environments, such as those characterized by high salinity, high humidity, and impact loading. Traditional steel used in offshore platforms struggle to effectively balance the requirements of lightweight and mechanical properties. In contrast, Fe-Mn-Al-C lightweight high strength steels have emerged as highly promising alternative materials due to their lower density (10%-20% lighter than traditional steel), excellent strength and good weldability. Currently, under the "rolling + solution treatment" process route, the appropriate design of Al element content plays a crucial role in enhancing the properties of Fe-Mn-Al-C lightweight high strength steels. By employing characterization techniques, including optical microscopy(OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD), the influence of Al element content on the microstructure and mechanical properties of the experimental steels during the staged preparation process was investigated. The results indicate that when the mass fractions of Al element are 5%, 8% and 12% respectively, the microstructure of the experimental steels after rolling treatment consists of austenite and ferrite phases. As the Al content increases, the morphology of austenite transitions from equiaxed to acicular, and the proportion of high-angle grain boundaries increases from 57% to 89%. When the Al element mass fraction is 8%, the rolled experimental steel exhibits the optimal strength-ductility balance, with a tensile strength of 747 MPa and an elongation of 28.3%. After solution treatment at 1 000 ℃, the comprehensive properties of the experimental steel with Al element mass fraction of 8% are further enhanced, achieving a tensile strength of 701 MPa, yield strength of 612 MPa, and elongation of 38.3%. Its fracture surface displays characteristic dimple patterns indicative of ductile fracture. Through optimization of Al element content and process control, precise design of the dual-phase microstructure in Fe-Mn-Al-C lightweight high strength steels have been realized, laying theoretical foundations for developing marine engineering steels.

In view of the problem of surface and internal cracks that were prone to occur when producing 304L austenitic stainless steel plate with high Cr and Ni content using continuous casting billets,effect of chemical composition and heating process on the microstructure and thermal plasticity of hot rolled 304L austenitic stainless steel plate was investigated. The results showed that with increasing of Ni element content, the ferrite transformation to austenite are more difficult of 304L austenitic stainless steel, and result in more large-sized ferrite phases distributed continuously at a certain angle to the rolling direction in the hot rolled plate, which deteriorated the bad thermal plasticity of the plate and formation internal cracks. With decreasing of Cr element content, the ferrite content is reduced, and result in the size and quantity of large-sized ferrite that is continuously distributed at a certain angle to the rolling direction in the plate and the thermal plasticity of the plate is improved further more. Higher heating temperature and holding time of the casting billets accelerated the transformation of residual ferrite to austenite during the hot rolling process, thereby the occurrence of large-sized ferrite that is continuously distributed at a certain angle to the rolling direction in the hot rolled plate is reduced, and the thermal plasticity of the plate is improved.

This cost-effective stainless steel composite plate integrates the excellent hydrogen resistance of stainless steel with the high strength and wear resistance of low-alloy steel. Such composites offer a promising approach for developing commercial materials with superior resistance to hydrogen. However, the influence of the cladding microstructure on the hydrogen resistance of stainless steel/carbon steel composite plates remains poorly characterized. Consequently, base materials exhibiting contrasting deformation resistance (Q235 vs. 700L) were employed to control the cladding microstructure. This work characterizes the microstructure of stainless steel and carbon layers in 316L/Q235 and 316L/700L composite plates and evaluates its impact on hydrogen resistance. Results show that the 316L cladding layer in the 316L/700L composite plate preferentially recrystallizes, reducing its hydrogen embrittlement susceptibility index from 9.8% of 316L/Q235 composite to 5.9%. Recrystallization control elevated ∑3 twin boundary density in 316L cladding (316L/700L) from 0.79% of 316L/Q235 composite to 19%. This microstructure simultaneously impedes hydrogen crack initiation through twin boundary resistance and suppresses propagation by disrupting random grain boundary networks, collectively enhancing hydrogen embrittlement resistance.

In recent years, low energy consumption and low cost CSP short-process technology has been rapidly developed in Chinese iron and steel industry. However, compared with the traditional hot rolling process, the yield strength of CSP products is higher, which is extremely unfavorable for the formability of steel used in stamping.Therefore, the dynamic recrystalization behavior and cooling phase transformation of austenite of DC04 cold-rolled deep-drawing steel were analized through thermal simulation experiments, and the dynamic recrystalization model was established, which was used to guide the production practice of CSP production line, and the cold-rolled deep-drawing steel sheet with rich {111}texture was successfully trial-produced. The experimental results show that the dynamic recrystallization model of DC04 cold-rolled deep-drawing steel in CSP production line isZ=ε·exp[295.21/(RT)]. The judgment conditions of dynamic recrystallization are Z=1.94×10¹⁰exp(61.34ε_c) and Z=2.57×10⁸exp(24.99ε_s).Based on the 3D dynamic recrystallization diagram, it is determined that the thermal deformation of the strip at F₃ stand in actual production is in the austenite part recrystallization region. In the trial production using F₃ stand bypassing scheme, the microstructure of the obtained strip is mostly equiaxed grain with low strength. However, the grains of the strips produced by the conventional rolling process of F₃ stand non-bypassing scheme,the grains contact with each other at sharp or right angles, and the strength is about 30 MPa higher than former. The F₁and F₂ stands of CSP production line adopt high temperature, high reduction rate and low deformation rate rolling, which can make the strip microstructure undergo complete dynamic recrystallization, complete the transformation of columnar crystal to equiaxed crystal, and avoid the generation of mixed crystal. The performance of cold-rolled sheets produced using a 70% cold rolling cumulative reduction, bell annealing at 710 ℃, and the F₃ stand by passing and non-bypassing schemes were compared. The cold-rolled sheets produced with the F₃ stand bypassing scheme showed little difference in strength compared to those produced with the conventional non-bypassing scheme, but exhibited higher elongation A₈₀ and r values reaching 45% and 2.1. Additionally, the proportion of the favorable texture component {111} was higher, resulting in excellent deep-drawing performance.

To reveal the influence of different solid solution temperatures on the microstructure of Invar 36 alloy and explore the optimal process window for obtaining good strength plasticity matching, the average grain size, degree of recrystallization, formation of annealing twins, and changes in grain boundary orientation of Invar 36 alloy hot rolled plates were studied at solution treatment temperatures of 650-900 ℃, and the influence of microstructural characteristics on their mechanical properties was analyzed. The results show that the microstructure of hot rolled Invar 36 alloy was mainly characterized by flattened and elongated fibrous deformation structure, and the recrystallization ratio of the alloy after solid solution treatment increased from less than 20% in the hot rolled state to over 70%. As the solid solution temperature increased from 650 ℃ to 900 ℃, the uniformity of the grain size was greatly improved, with the majority of the grains being equiaxed austenitic structures without distortion. The grain size increased from 21.3 μm to 31.85 μm, and the proportion of high-angle grain boundaries gradually increased. The percentage of annealing twins in the total grain boundaries had increased by 13%. At a solid solution temperature of 750 ℃, the alloy exhibited tensile strength similar to that of the hot rolled state, with a total elongation after fracture of 40%. The toughness of the tensile fracture surface was characterized by tearing, demonstrating good comprehensive mechanical properties.

Aiming at the problems of cold cracking, heat affected zone softening,toughness decreasing, and residual stress in the welding of high-strength steel for automobiles, the Q960E automobiles high-strength steel has been taken as the research object, the laser composite welding test method has been used to study the effects of different heat treatment parameters such as preheating temperature, quenching temperature, and tempering temperature on the microstructure and mechanical properties of the welded joint. The research results indicate that the welded joint of Q960E high-strength steel consists of a welding base material, a weld seam, a fusion zone, an overheated zone, and a normalized zone, at preheating temperature of 100 ℃, quenching temperature of 900 ℃, and tempering temperature of 400 ℃, the base material is tempered martensite, the weld seam is fine bainite, the fusion zone is fine bainite and coarse martensite, the overheated zone is coarse bainite, and the normalized zone is a recrystallized crystal composed of bainite and martensite. As the quenching temperature increases, the tensile strength and yield strength of the welded joint show a trend of first increasing and then decreasing, and the elongation at break shows a trend of first decreasing and then increasing. As the tempering temperature increases, the tensile strength and yield strength of the welded joint show a gradually decreasing trend, while the elongation at break shows a gradually increasing trend. When the preheating temperature is 100 ℃, the quenching temperature is 900 ℃, and the tempering temperature is 400 ℃, the tensile strength of the welded joint is 1 150 MPa, the yield strength is 1 075 MPa, and the elongation at break is 10.56%, the comprehensive performance is superior, and it is the best heat treatment processes for Q960E automotive high-strength steel.

In order to obtain the optimal heat treatment process for offshore FH790 heavy plate, four cooling methods, namely water cooling, oil cooling, air cooling, and sand cooling, were used to represent different cooling rates to study and analyze the effect of different quenching cooling rates on the microstructure and mechanical properties of FH790 offshore steel. The analysis results show that the microstructure of experimental steel under water cooling and oil cooling is bath martensite. The microstructure of the experimental steel under air cooling is bath martensite mixed with granular bainite, and granular bainite and ferrite generated under sand cooling. As the cooling rate decreases, the strength of the experimental steel gradually decreases, and the elongation does not change much. The -60 ℃ low-temperature impact toughness is highest under oil cooling, with an average impact energy of 150 J. The strength is high under water-cooling, but the toughness is poor. The strength, plasticity, and toughness of air cooled and sand cooled experimental steels are relatively poor. Therefore, to ensure the uniformity of mechanical properties at different positions in the thickness direction of the FH790 offshore heavy plate, it is required that the cooling rate at the core position be greater than that of the oil cooling method by 40 ℃/s during quenching, so as to obtain the ideal microstructure of the core and improve the low-temperature impact toughness of the FH790 offshore heavy plate core.

Hot-rolled galvanized high hole expansion steel possesses both high formability and high corrosion resistance, which can significantly improve the service life of automotive chassis parts to meet the requirement of new energy vehicles. However, due to the addition of annealing and galvanizing processes, the precipitation behavior and changes in microstructure and mechanical properties of hot-rolled galvanized sheets are more complex compared to pickled sheets, and require further research. The effect of coiling temperature on microstructure and properties of 580 MPa grade Nb microalloying hot-rolled galvanized high hole expansion steel was studied by using optical microscope (OM), scanning electron microscope (SEM), tensile testing machine and forming testing machine. The results show that compared with high-temperature coiling at 570 ℃, the hot rolled steel produced by low-temperature coiling at 450 ℃ had relatively smaller grain size and remained more defects such as dislocations, resulting in higher energy storage. Therefore, during subsequent annealing process, the driving force for recrystallization was greater and there were more nucleation positions, making the galvanized steel has a finer grain structure. Low temperature coiling suppressed Nb precipitation during the hot rolling stage, allowing most Nb to dissolve in the ferrite matrix or be in a critical precipitation state. In the subsequent annealing and galvanizing process, more small-sized and uniformly distributed Nb-containing precipitates were obtained, which not only contributed positively to yield and tensile strength, but also significantly improved the hole expansion rate. The galvanized sheet used the hot rolled strip of coiled at 450 ℃ possessed excellent mechanical properties and local formability. Its longitudinal yield strength, tensile strength and total elongation was 503 MPa, 602 MPa and 20.5%, respectively. The average hole expansion rate reached 95%.

In order to develop ultra-high-strength prestressed steel strands with higher development intensity and superior comprehensive performance, using 87Mn wire rod as the raw material, the evolution of the microstructure and properties of the steel wire during the nine-pass drawing process were studied, revealing its strength-ductility mechanism. The results showed that the equiaxed pearlite structure was elongated and refined along the drawing direction with the drawing process, showing obvious directionality, and finally forming a typical fibrous drawing structure. The dislocation density inside the steel wire increases with the increase of drawing strain, and the growth rate is faster in the early drawing stage, from 2.35 × 10¹⁴ m^-2 to 4.59 × 10¹⁴ m^-2, and the growth rate of dislocation density gradually slows down in the middle and late drawing stages. After 9 passes of drawing, the tensile strength of 87Mn wire rod increases from 1 170 MPa to 2 085 MPa, and the elongation decreases from 12.56% to 7.20%. The change of mechanical properties during the drawing process is mainly divided into three stages:in the early stage of drawing, dislocation strengthening and boundary strengthening work together, and the strength contribution of dislocation strengthening growth is obvious, from 189 MPa to 460 MPa. At this stage, the strength of the steel wire increases with the increase of the drawing strain, and the plasticity decreases significantly. In the middle of drawing, dislocation strengthening and boundary strengthening work together. The strength contribution of boundary strengthening is significant, from 600 MPa to 806 MPa, and the growth rate of dislocation strengthening decreases. At this stage, the plastic decline trend of steel wire slows down and the strength continues to increase. In the late stage of drawing, a large number of cementite dissolved, and the strength contribution of solid solution strengthening was enhanced to 149 MPa. At this stage, the combined effects of boundary strengthening, dislocation strengthening and solid solution strengthening worked together. The strength growth rate of steel wire was obviously accelerated, and the plasticity decreased slightly.

Driven by the "dual-carbon" strategy, the transformation of 82B wire rod production processes from traditional long-process to electric furnace short-process has become inevitable. However, there is still a lack of systematic research on the causes of abnormal martensite structures near the core of wire rods produced by short-process technology. To address this, this study investigated the causes of abnormal martensite structures in 82B wire rods produced by recycled steel materials+electric furnace short-process through metallographic observation, quantitative analysis of macro/micro-segregation, and thermal simulation tests, combined with comparative analysis of 82B wire rod structures produced by traditional long-process technology. The results show that the segregation levels of Mn and Cr elements near the core of short-process 82B wire rods are more severe than those of long-process 82B wire rods. This segregation, along with the higher residual Ni content, shifts the C-curve of 82B steel to the right, reduces the critical cooling rate for martensite formation from 7 °C/s to 3 °C/s. As a result, the martensite content of short-process 82B wire rods under the same cooling rate is higher than that of long-process 82B wire rods. Based on experimental results, process adjustments were applied, including increasing the soaking temperature of billets from (1 120±10) ℃ to (1 160±10) °C and maintaining the heating and soaking holding times at 100-120 min. These adjustments significantly improved macro-segregation and suppressed the formation of abnormal martensite structures.

For large size hot rolled H-beams with a web height H≥1 000 mm and a flange thickness h≥40 mm, the rolling load is relatively high in production and it is difficult to improve the product's microstructure and properties through traditional low-temperature and large reduction technology on existing production equipment. In order to achieve the localization of large size hot rolled H-beams, thermal simulation experiments, optical microscopes, scanning electron microscopes, transmission electron microscopes, tensile and impact tests were used to study the effect of rolling temperature on the microstructure and properties of Q420 grade HN1 109 mm×461 mm×21 mm×40.5 mm H-beam flanges. The results show that when the universal rolling temperature is reduced from1 000 ℃ to 900 ℃, the flange yield strength increases, the impact toughness first decreases and then increases, and the grain size is refined to a certain extent, the amount of V carbonitride precipitation increases, and its size gradually decreases. When the universal rolling temperature is controlled at 900 ℃ or 1 000 ℃, the microstructure and properties of Q420 grade large size hot rolled H-beam match well.

In order to satisfy the green and efficient rolling production, the double stands reversible breakdown rolling process was proposed, two layouts of double stands individual and tandem reversible breakdown rolling were analyzed and compared. In view of the high production flexibility of breakdown rolling for special-quality steel, it will be more advantageous to choose the double stands individual reversible layout. Based on the above, the roller grooving configuration mode and rolling pass arrangement for H-H double stands individual reversible breakdown rolling process were analyzed and discussed, the rolling temperature and parameters of typical steel grades in various pass were calculated and checked. Meanwhile, the deformation permeability in various pass of breakdown rolling was simulated and calculated by FEM software-ABAQUS. The results indicated that the spare parts of breakdown rolling mills could be unified if BD₁ and BD₂ rolling mills were selected by the same roller grooving configuration and the same mill type. Meanwhile, it was provided with good production flexibility that the roller systems of two rolling mill stands were exchanged by each other when the roller grooves should be turned again, or two rolling mill stands were chosen one to use and the other to maintain. For the production of typical steel grades, the overall temperature drop of the breakdownrolled billet could be controlled within 100 ℃ and the final rolling temperature could be maintained above 950 ℃, so that the hot processing temperature window control of corresponding steel grades and their treatment requirements of subsequent procedures could be satisfied, and also the temperature drop of delivering between two mill stands would be controllable. Moreover, the parameters of main motors and their overload capacity could be satisfied with the requirements of rolling parameters for typical steel grades. The good deformation permeability at the core of the billet could be obtained by applying larger reduction after the accumulated deformation by previous two passes. For the billet with the dimension of 300 mm×390 mm, the fine deformation uniformity for the small rolled billet with the cross section of 150 mm×150 mm could be achieved by breakdown rolling of more than 5 accumulated reduction ratio. Finally, the typical process layout of double stands reversible breakdown rolling for special-quality steel was introduced, and the application prospect for the breakdown rolling process mentioned above was forecasted in the base of wire rod and straight bar for special-quality steel.

Aiming at the problems of long roll changing time and low reliability of roll changing device in the continuous cold rolling production line at present,the structure and principle of work roll changing device of 4-high skin pass mill were introduced. Through comprehensive analysis of work roll changing steps and time sequence, some of the series operation steps were optimized to parallel operation or lap operation. It was found that the key factor affecting the work roll changing time was the running speed of AGC cylinder, and then the servo hydraulic system was optimized with constant power pump characteristics to improve the running speed of AGC cylinder and greatly reduce the work roll changing time. In order to improve the reliability and stability of the work roll changing device, the structure design, the selection of detecting elements and the installation accuracy of the device were optimized and improved. After adopting above measures, the work roll changing time is greatly compressed from more than 120 s to less than 90 s, and the operation reliability and stability are significantly improved, which lays a foundation for the continuous and stable production of the cold rolling production line and the improvement of product quality.

When the traditional 20-high rolling mill is used to expand the ultra-thin specifications during the rolling of silicon steel, the evaluation of equipment capacity and the selection of process parameters are based on experience and field trials. The lack of theoretical support leads to a high risk of equipment damage and high trial and error costs. At the same time, the product quality cannot be guaranteed after the specification expansion. In response to the above problems, fully considering the equipment and process characteristics of the 20-high rolling mill, combined with actual rolling data on site, and taking into account rolling stability and strip surface defect control, a 20-high rolling mill rolling capability evaluation model and technology was established and corresponding rolling ability accounting software was developed. This software can be used to accurately calculate rolling capacity of a 20-high rolling mill for silicon steel. It was applied to a cold rolling plant for silicon steel of a steel company. Through comparative analysis of the calculated minimum product thickness and the actual export product thickness, it was found that, under the premise of ensuring hourly output and considering rolling stability and defect control capabilities, the calculated minimum product thickness is smaller than the actual product thickness, and the product thickness specification has room for expansion in a thinner direction, which laid a theoretical foundation for the subsequent product quality control and rolling process development of high-grade non-oriented silicon steel and oriented silicon steel to expand to thinner specifications.

Microstructure analysis is an important analysis method in the research and development process of iron and steel materials. At present, it is mainly judged manually by experts with rich experience, which is time-consuming and easily affected by subjective consciousness. Therefore, an intelligent analysis method for microstructure based on the residual neural network structure is studied. By improving the residual network model, an improved residual network model based on transfer learning and a deep residual shrinkage network model based on the attention mechanism are proposed. Two different convolutional neural network models are used for verification on the microstructure test set of 20 steel materials. The experimental results show that the accuracies of the two models reach 95.36% and 95.79% respectively, with strong generalization ability, and the shortest average prediction time is only 1.66 s per image. The two models have certain advantages in the classification of microstructure features of iron and steel materials, realizing the automation and intelligence of microstructure type classification.

With the implementation of the "dual carbon" strategy, the steel industry has put forward higher requirements for optimizing the electricity consumption required for hot rolling of medium and heavy plates. Aiming to achieve quantitative calculation and prediction of electricity consumption during hot rolling of medium and heavy plates, a theoretical electricity consumption calculation model was constructed and a set of electricity consumption calculation software for hot rolling of medium and heavy plates was developed. This software can formulate the rolling schedule according to the produced steel and target demand and accurately calculate the electricity consumption for each pass of rolling manufacturing and the electricity consumption per ton of steel of a single slab, which can provide data support for steel enterprises to quantify the electricity consumption in the medium and heavy plate rolling process and optimize the rolling process. By comparing with the manufacturing data of the steel mill, the calculation results of the software are basically consistent with the actual situation when the thickness hit rate is almost 99%,the deviation of electricity consumption in each pass of roughing rolling is kept within 10%, and the deviation of electricity consumption per ton of steel inrouging rolling is -3.12%-3.86%, which can accurately reflect the electricity consumption of the plate in the rolling process. After verification, this software can provide data support for developing new steel grade and new processes, scheduling manufacturing, selecting the main motor of the rolling mill, and so on, which helps steel enterprises further achieve energy conservation and emission reduction, and create economic benefits for them.

To produce 9Ni steel plates with excellent microstructure, properties andplate shape in wide and thin gauge via a medium-heavy plate mill, the pack rolling process was investigated through experiments and numerical simulations. The chemical composition of 9Ni steel was designed, and the CCT curve of the tested steel was determined. Through pilot trials, 3 mm thick pack-rolled plates were obtained alongside suitable welding methods for stacking and spacer compound formulations. Finite element simulations were used to analyze stress and strain distribution patterns under different reductions to establish reasonable rolling schedules. Results indicate that uniform stress distribution at 10 mm reduction, while reductions greater than 15 mm caused non-uniform distribution with significant differences between the lower surface of the top plate and its upper surface.There is higher stress at edges but relative uniformity centrally. Industrial production on the medium-heavy plate mill implemented roughing rolling start at 1 150 ℃, finishing rolling start at 950 ℃, and finishing rolling end at 810 ℃. According to the CCT curve, cooling rates greater than 5 ℃/s yield fully martensitic microstructure. The Ac₁ and Ac₃ of the tested steel are 621 ℃ and 735 ℃, quenching temperature must exceed 735 ℃ for complete austenitization, while two-phase region quenching above 621 ℃ achieves reversed austenite. The 5 mm thick wide and thin gauge plates exhibited excellent comprehensive mechanical properties with flatness less than 3 mm/m, meeting standards and customer requirements.

In order to improve the qualification rate of ultrasonic testing for continuous casting extra thick slab, reduce the production cost of heavy plates, and prevent quality defects such as "white spots", "hydrogen embrittlement", and "point segregation", and overcome the difficulties of traditional experimental methods for detecting hydrogen diffusion in extra thick slab, a numerical simulation method was used to study the process of reducing hydrogen concentration in steel to a safe range through stacking cooling, and good results were achieved. Due to the thickness of the continuous casting slab reaching 400 mm, it takes a long time for hydrogen to diffuse from its core to the surface. Ordinary stacking slow cooling cannot meet the hydrogen removal effect of the slab. Therefore, using slow cooling pits and slow cooling pit heating methods for stacking slow cooling can further improve the insulation effect of the slab and make the hydrogen diffusion of the slab more sufficient. Therefore, a mathematical model for hydrogen diffusion in slab was established based on three stacking methods. The effects of ordinary stacking, slow cooling pit stacking, and slow cooling pit heating on hydrogen diffusion in slab were compared, and the hydrogen content and hydrogen removal rate at each position of the slab were obtained. Under the three stacking modes of ordinary stacking, slow cooling pit stacking, and slow cooling pit heating, the hydrogen removal rates of the bottom slab were 58.93%, 67.63%, and 71.98%, respectively. The slow cooling pit stacking method and slow cooling pit heating method are not only more conducive to the hydrogen diffusion in extra thick slab but also can make the hydrogen diffusion on the upper and lower surfaces of the extra thick slab more uniform.

Based on a hot-dip galvanized CR3 sheet with a Zn-1.5%Al-1.5%Mg zinc-aluminum-magnesium coating (CR3-ZM70) and a pure zinc-coated CR3 sheet (CR3-Z70), the coating phasesare analyzed and the forming performance, weldability, paintability, adhesion properties, and corrosion resistance in service are experimentally analyzed. The feasibility of replacing pure zinc-coated sheets with Zn-Al-Mg-coated sheets in automotive body applications is assessed. The results demonstrate that the Zn-Al-Mg coating consists of pure zinc, zinc-magnesium binary, and zinc-aluminum-magnesium ternary phases, with all phase grain sizes 50 μm or small and significantly smaller than those of the pure zinc coating. The addition of Al and Mg increases surface oxide formation on the Zn-Al-Mg coating. Its nano-hardness exceeds that of pure zinc coating by over 33%. The surface friction coefficient of CR3-ZM70 is 27% lower than CR3-Z70, and it exhibits better friction stability after repeated tests. CR3-ZM70 shows 72% less stamping scale-off compared to CR3-Z70. The welding current window width and nugget size are comparable between the two coatings. Both achieve equivalent electrocoating performance, with phosphating film grain sizes 10 μm or small. However, CR3-ZM70 exhibits lower cohesive fracture performance in some structural adhesives and spot welding pastes. Under identical neutral salt spray conditions, CR3-ZM70 demonstrates three times the corrosion resistance of CR3-Z70, with significantly better cyclic corrosion resistance in post-electrocoating exposed substrate tests. The experimental results confirm that Zn-Al-Mg-coated sheets outperform pure zinc-coated sheets in forming performance and corrosion resistance, while showing comparable weldability and paintability. However, their adhesion property is inferior to that of pure zinc-coated sheets. Thus, Zn-Al-Mg coatings are viable substitutes for automotive body applications but cannot fully replace pure zinc coatings.

Aiming at the problem of severe edge-cracking defects of ultra-low carbon steel hot rolled strips for cold rolled sheets of automotive produced by Qiansteel, where the occurrence rate is as high as 30%. The severe burr defects would appeared after cold rolling and difficult to polish clean lead to iron chip adhesion on furnace of rolls in the continuous annealing furnace area, and it formed scale tumors and caused periodic indentation defects that seriously affect the surface quality of automotive sheets. Through microscopic analysis of the defects, a large number of oxide particles and copper element enrichment were found around the defects, tracing the root cause could be backed to before slab heating, i.e. the precipitation and enrichment of copper elements on the slab surface were the fundamental cause of the edge-cracking defects. Combining the macroscopic law that the occurrence rate of edge-cracking defects in flame-cleaned (abbreviated as mechanical cleaning) slabs is as high as 30%, while non-mechanical cleaning slabs show no such defects, inspections on the quality of mechanical cleaning slabs and mechanical cleaning equipment were conducted. It was determined that the root cause of edge-cracking defects is the local burning damage of copper foil in the narrow-face burner gap during mechanical cleaning of slabs, leading to copper element infiltration into the narrow face of the slab and inducing "copper embrittlement"-type defects. Effective solutions to the edge-cracking defects were achieved through measures such as controlling mechanical cleaning burners and optimizing the heating and scale removal processes in the hot rolling process.

Regarding the problem of large cutting loss caused by clustered fine line defects at the edge of dual-phase steel base material for cold rolling, macroscopic and microscopic morphology of defects are analyzed and the hot rolling production process is investigated. The causes of clustered fine line defects at the edges of dual-phase steel base materials for cold rolling have been identified, and targeted improvement measures have been proposed. The results show that the formation of clustered fine line defects at the edge of dual-phase steel base materials for cold rolling is due to the process of roughing rolling. Pit defects are formed when the slab contacts with the vertical roll. Pit defects extend along the rolling direction during the rolling process, forming fine line defects and gradually flip up to the upper surface as the metal flows at the edges and corners of the slab during roughing rolling. After being rolled by a flat roll, edge clustered fine line defects are formed on the surface of dual-phase steel base material for cold rolling. By shortening the vertical roll replacement cycle and adjusting the roughing rolling mode, the clustered fine line defects at the edge of dual-phase steel base material for cold rolling are eliminated, and the quality of the dual-phase steel edge is improved.

Q355D angle steel is the key material of transmission tower under extreme cold conditions, and its low temperature impact toughness is strictly required. The microstructure and fracture morphology of Q355D angle steel specimens with different chemical compositions were analyzed by stereomicroscope (SM), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), optical microscope (OM) and transmission electron microscope (TEM). The initiation and propagation behavior of impact cracks were clarified. The results show that the content of carbon and nitrogen in the chemical composition of Q355D angle steel will significantly affect its low temperature toughness. The high content of carbon and nitrogen elements will cause the increase of cluster pearlite structure and the increase of interstitial solute dissolved in the lattice of metal materials, resulting in the deterioration of plasticity and the increase of brittleness of angle steel. In addition, there is also a banded structure in the sample with higher carbon content, which also leads to the deterioration of the plasticity of the angle steel. In the samples with unqualified impact properties, the phenomenon of Al₂O₃ inclusions and second phase particles enrichment and growth is also found. Due to the above reasons, the brittle fracture of the samples is caused. Therefore, the mass fraction of carbon in Q355D angle steel should not exceed 0.16% to reduce the content of pearlite and improve the segregation of pearlite clusters. In its production, deoxidizer should be accurately added and calcium treatment process should be optimized to avoid excessive aluminum residue reacting with oxygen to form Al₂O₃ inclusions. Continuous casting adopts digital electromagnetic stirring process and soft reduction process to improve banded structure and central segregation. After adopting the above measures, the stable production of Q355D angle steel is realized.