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.
The hot working properties of superautenitic stainless steel exhibit sensitivity of variations in process parameters such as deformation temperature,strain,strain rate.In order to study the influence of deformation temperature and deformation rate on the hot working performance of S32654 super austenitic stainless steel, a hot tensile test was conducted. The temperature range for the hot tensile test was 1 050-1 200 ℃, and the strain rate ranged from 0.01 s-1 to 10 s-1. The fracture morphology and microstructure evolution of the super austenitic stainless steel under different deformation conditions were analyzed using XRD and EBSD. The results indicate that the S32654 super austenitic stainless steel exhibits good hot plasticity under the deformation conditions of 1 150 ℃ to 1 200 ℃ and 1 s-1to 10 s-1, while its hot plasticity is poor under the conditions of 1 050 ℃ to 1 100 ℃ and 0.01 s-1to 0.1 s-1. Under the deformation condition of 1 200 ℃ and 10 s-1, the peak stress, maximum true strain and reduction of section are 269.5 MPa, 0.4 and 55.6%, respectively, showing the best thermoplastic property. Observation of the fracture morphology of samples reveals that the S32654 super austenitic stainless steel primarily exhibits intergranular brittle fracture in the low temperature and low strain rate regions, while it shows ductile fracture in the high temperature and high strain rate regions. EBSD microstructural analysis indicates that the occurrence of dynamic recrystallization at high temperatures and high strain rates hinders the formation and propagation of cracks. Additionally, a significant number of annealing twin boundaries are present in the recrystallized grain regions of the S32654 super austenitic stainless steel, which is related to its dynamic recrystallization mechanism being predominantly discontinuous dynamic recrystallization.
To formulate and optimize the hot working process for 18Ni (200) maraging steel, the hot deformation behavior of 18Ni(200) maraging steel was investigated using an MMS-200 thermal simulation testing machine. A hot processing map was established, and the microstructure evolution mechanism was analyzed. The results show that under the conditions of deformation temperatureT=850-1 100 ℃, strain rate=0.01-10 s-1, and a maximum true strain ε=0.6, the flow stress of 18Ni (200) maraging steel decreases with increasing deformation temperature or decreasing strain rate, while the power dissipation efficiency gradually increases, leading to more complete dynamic recrystallization (DRX). When the true strain reaches 0.2, the area of flow instability zones is minimized. The flow instability zone is mainly distributed in the high temperature and high strain rate region. The complete recrystallization region of 18Ni (200) maraging steel was determined as follows deformation conditions:T=950 ℃、≤0.1 s-1;T=1 000 ℃、≤10 s-1;T=1 050 ℃、≤10 s-1;T=1 100 ℃、≤10 s-1.
For weathering steel, the structure and nature of the dense surface rust is a key factor affecting the corrosion resistance of the material. In this paper, the effects of three alloying elements, Cr, Mn, and Cu, on the corrosion resistance of Fe3O4 were systematically analyzed using first-principle calculations. The results show that the addition of Cr, Mn and Cu elements can effectively protect the Fe atoms on the surface of Fe3O4, reduce the spacing between the surface atoms and the subsurface atoms, and impede the corrosion of the surface Fe atoms by external corrosive ions. Among them, the presence of Cu atoms can effectively reduce the surface energy of Fe3O4and increase the stability of the Fe3O4 structure; Cr and Mn atoms can promote the separation of H2O molecules on the surface, resulting in the formation of FeOOH structure with good corrosion resistance. In addition, Cr, Mn and Cu can effectively reduce the effect of H2O or Cl-+H2O corrosive environment on the chemical state of Fe atoms on the surface of weathering steel , reduce the adsorption of Fe3O4 on Cl- and H2O, and finally improve the corrosion resistance of Fe3O4.
In view of the problem of color difference defects on DH980 hot rolled strip after pickling, isothermal oxidation experiments were conducted to analyze the morphological characteristics of these defects and the formation mechanisms of intergranular oxidation, thereby identifying the root cause of the defect. The results indicate that the primary cause of color difference defects on pickled DH980 strips is intergranular oxidation within the hot-rolled coils. In following tension leveling, the bonding strength at intergranular oxidation sites decreases, leading to crack initiation. During the scale removal process in picking, pickling cracks appear at these intergranular oxidation regions, resulting in macroscopic color difference defects. Notably, slow cooling rates in the holding pit exacerbate intergranular oxidation depth. Isothermal oxidation experiments at 600-1 000 ℃ revealed that DH980 strip exhibits varying degrees of intergranular oxidation within the 700-1 000 ℃ range. Intergranular oxidation disappears at 600 ℃, with 800 ℃ identified as the critical temperature for maximum susceptibility. The depth of the intergranular oxide layer correlates closely with the competitive mechanism between internal and external oxidation. By optimizing coiling temperature and adjusting pickling parameters, the adverse effects of intergranular oxidation can be mitigated, effectively suppressing pickling color difference defects on DH980 hot rolled strips.
A 700 MPa grade engineering machinery steel with Ti element mass fraction of 0.12% was prepared using a process route involving thermo-mechanical rolling and off-line tempering. Its microstructure and property regulation to determine the optimal tempering process were researched. The experimental steel billet was heated 1 220-1 250 ℃, followed by roughing and finishing rolling on a 2 250 mm hot rolling line, with the finishing rolling temperature controlled between 840 and 870 ℃. The hot-rolled strip was then subjected to laminar cooling and coiled at temperatures ranging from 580 to 610 ℃. After cooling, the coil was cross-cut and subjected to off-line tempering using a roller hearth open-flame furnace, with tempering temperatures controlled at 620, 650, and 680 ℃. The microstructure, precipitates, and mechanical properties of the plate after tempering were studied using optical microscopy, scanning electron microscopy, and transmission electron microscopy. The results showed that the tempering treatment led to the decomposition of M/A islands in the hot-rolled microstructure and a significant reduction in dislocation density within the grains. Additionally, a large number of fine, dispersed nano-scale precipitates were formed, resulting in an increase in yield strength by 105-139 MPa and tensile strength by 45-82 MPa compared to the hot-rolled state. As the tempering temperature increased, the internal stress in the plate was fully released, and the impact toughness was significantly improved. The experimental results indicated that the plate tempered at 650 ℃ exhibited the best comprehensive performance, effectively meeting the usage requirements for engineering machinery vehicles.
To enhance the scale explosion resistance performance of hot rolled enamel steel while reducing production costs and carbon emissions, the effects of different hot rolling and enameling processes on the microstructure, mechanical properties, and hydrogen permeation behavior were investigated through hot rolling experiments and enamel-firing tests. The results indicate that the microstructure of hot rolled experimental steels primarily consists of irregular ferrite and cementite. Lower final cooling temperatures induced granular bainite formation, where M/A islands within grains provided nucleation sites for recrystallization during enameling, generating numerous fine ferrite grains. This grain refinement significantly inhibited yield strength reduction through fine-grain strengthening. After enameling, the second-phase particles mainly comprised Ti4C2S2 with minor TiC and TiN particles. Lowering the finishing rolling temperature increased irreversible hydrogen trap density, effectively enhancing scale explosion resistance performance of experimental steel. Specimens enamel-fired at 930 ℃ demonstrated yield strengths exceeding 385 MPa and hydrogen lag times (tL) over 6.16 min, while those enamel-fired at 870 ℃ achieved yield strengths above 437 MPa with tL exceeding 8.56 min. Optimized hydrogen storage performance was achieved through lower final cooling temperatures, low-temperature enameling processes, and utilization of Ti4C2S2 as predominant irreversible hydrogen traps.
To address the automotive industry's demand for low-cost, high-performance, and lightweight wheel steels, an 800 MPa grade wheel steel has been developed. The chemical composition was designed utilizing a C-Mn-V-Nb-Cr alloy system, with an emphasis on optimizing vanadium utilization in Panzhihua-region resources. A thermo-mechanical controlled process (TMCP) was implemented, involving the following parameters: slab heating at 1 200 ℃ for 2 h holding; roughing rolling at 1 100-1 000 ℃ with three passes (compression ratio 2.5); finishing rolling at 860-830 ℃ with six passes (compression ratio 5.7); followed by 15 s air cooling to 750 ℃, then water quenching to 430 ℃, and final air cooling to room temperature. Microstructural characterization and mechanical testing revealed excellent properties of the developed steel which yield strength is 690-720 MPa, tensile strength is 860-870 MPa, yield-to-tensile ratio is 0.80-0.88, and elongation is 20%-22%. Notably, the impact energy at temperatures ranging from 20 ℃ to -60 ℃ consistently exceeded 140 J. Both cold bending and hole expansion capabilities meet the stand requirements for 800 MPa grade wheel steels.
A new processing route including strip casting, cold rolling and annealing was proposed to solve the problems of low yield and productivity in traditional processing route for Invar alloy thin sheet. The optical microscopy, EBSD, EPMA and SEM were used to investigate the microstructure evolution and the mechanical properties were measured. The results showed that the cracks, intergranular oxidation and macroscopic segregation of Ni were absent in the as-cast strip. The microstructure was mainly composed of coarse and columnar austenite grains. The 0.5 mm thickness and 0.7 mm thickness cold rolled sheets were manufactured by using new processing route. Lots of twins were formed in the annealed sheets. The yield strength of the annealed Invar alloy sheets was in the range of 251-281 MPa, while the tensile strength was in the range of 420-445 MPa and the elongation was higher than 30%. The tensile fracture surfaces exhibited typical ductile fracture morphology. The mechanical properties of the Invar alloy annealed sheets were similar to those produced by traditional processing route.
Medium-temperature copper-containing CGO silicon steel is widely used in the manufacture transformer core, and its performance improvement is of great significance for energy saving and emission reduction. Reducing strip thickness is an effective way to reduce iron loss, but with the increase of cold rolling reduction rate, the number of Goss grains in cold rolled strip is reduced, and the inhibitor ripening is accelerated, which brings challenges to the industrial upgrading of CGO silicon steel. The microstructure, texture and inhibitors of intermediate and high temperature annealing sheets of CGO silicon steel under different processes were characterized by EBSD and SEM. The results indicate that the conventional process, which controls the reduction rate of the second cold rolling at 55% to 60%, is no longer suitable for the requirements of thin gauge strip. Under the conventional process, the area fraction of Goss grains in the intermediate annealed plates is 2.81%. After primary recrystallization, the area fraction of Goss grains decreases to 0.742%, and the secondary recrystallization initiation temperature is 940 ℃. This leads to the abnormal growth of deviated Goss grains as well, resulting in a magnetic induction intensity B8 of 1.682 T. By adjusting the reduction rates of the first and second cold rolling processes from 78% and 54% to 72% and 64%, respectively, the area fraction of Goss grains was significantly improved. The proportion of Goss grains in the intermediate annealed sheets increased to 3.84%, and after primary recrystallization, the proportion of Goss grains further rose to 1.47%. Under unchanged heat treatment parameters, this adjustment promoted more sufficient secondary recrystallization of Goss grains, resulting in an increase in the magnetic induction intensity B8 of the finished sheets to 1.836 T. By utilizing the wide process window of the secondary cold rolling method, a nitriding process was introduced during the intermediate annealing stage. This addition did not affect the microstructure or texture of the CGO silicon steel but facilitated the precipitation of AlN particles during the secondary recrystallization process. As a result, the onset of secondary recrystallization was delayed, and the secondary recrystallization temperature was increased to 980 ℃. This ensured more accurate orientation of abnormally grown Goss grains, leading to a further improvement in the magnetic induction intensity B8 of the finished sheet to 1.900 T. By regulating cold rolling parameters and introducing a nitriding process during intermediate annealing, high-performance medium-temperature copper-containing CGO silicon steel was successfully prepared. This achievement provides a new approach for enhancing the performance of CGO silicon steel while achieving energy savings and cost reductions.
In order to reveal the evolution law of the microstructure of complex phase steel with improved formalility during annealing, thermal simulation experimental machine and microstructure characterization methods such as electron probe microanalysis (EPMA), electron backscatter diffraction (EBSD), X-ray diffraction (XRD) were used to study the kinetics of bainite transformation in experimental steel. The influence of annealing process parameters such as soaking temperature, over aging temperature and over aging time on microstructure evolution was revealed. The results show that during the bainitic insulation phase transformation process, the bainitic phase transformation occurs when the over aging temperature is 350-400 ℃, and the completion time of the phase transformation is 84-131 s. When the over aging temperature is higher than 450 ℃, there is basically no bainitic transformation, and martensitic transformation occurs during the cooling process after the insulation is completed. During the simulated continuous annealing process, as the soaking temperature increases, the content of bainite in the experimental steel increases, while the content of ferrite and residual austenite gradually decreases. With the increase of over aging temperature, the content of bainite decreases, while the content of residual austenite first increases and then remains unchanged. The grain size ranges from 0 to 1.5 μm, mainly concentrated below 1 μm. As the over aging time increases, the bainite content in the experimental steel remains basically unchanged, while the residual austenite content increases. When the continuous annealing process is carried out at a soaking temperature of 880 ℃, over aging temperature of 400 ℃ and over aging time of 600 s, the maximum volume fraction of residual austenite in the experimental steel is 10.83%.
Inaccurate rolling force settings during cold rolling can lead to excessive thickness deviations at the strip head after dynamic gauge changes (FGC), significantly reducing product yield. A genetic algorithm was used to sequentially optimize the friction coefficient model and deformation resistance model in the rolling force calculation. The optimized model demonstrates high precision, with calculated rolling force deviations below 4.82% across all rolling stands compared to actual measurements. Field trials confirmed significant improvements:the proportion of strips with head-end thickness deviations under 20 m increased from 38.8% to 55.8% after optimization, effectively enhancing cold rolled product yield.
Based on the application status of controlled rolling and controlled cooling process for hot rolled H-beams, combined with engineering examples, the application of controlled cooling technology in H-beam production was introduced in detail, including the layout of controlled cooling process, configuration of cooling modules, arrangement of cooling nozzles, and implementation plan of controlled cooling process. By combining the temperature model of cooling curve with the field measured data, it can effectively guide the implementation of H-beam controlled cooling process. This method significantly reduces the cooling non-uniformity of H-beams, ensures the dimensional and performance uniformity along the length direction of the products, avoids defects such as web waviness and cracks during cooling process, improves the comprehensive mechanical properties of H-beams, and reduces production costs. The research results can provide effective guidance for industrial production of H-beams.
To address the frequent wear of the intermediate roll bearing seat liner plate in a domestic cold tandem rolling mill, the wear patterns of the liner plate were analyzed firstly, andthe primary influencing factors were identified as rolling kilometers, normal pressure on the liner plate, mill vibration and liner plate material. Further, by investigating the mechanisms through which these factors affect liner plate wear and combining actual wear data under different production processes and operating conditions, a mathematical model was established to correlate rolling kilometers and mill vibration with liner plate wear. Based on the classical Archard wear theory and an analysis of equipment and process parameters of the rolling mill, a calculation method for determining the normal pressure on the liner plate during production was derived. A wear prediction model incorporating normal pressure and material of the liner plate was subsequently developed. By integrating the effects of rolling kilometers, normal pressure, mill vibration and material, a comprehensive prediction model for liner plate wear was proposed. An objective function for regression of model coefficients was constructed, and the regression coefficients for each influencing factor were determined. The model was applied to a domestic cold tandem rolling mill, and wear predictions under different production processes were compared with actual measurements. Results indicate that the model achieves high accuracy during the early and middle stages of liner plate service, with prediction deviations gradually increasing as rolling kilometers accumulate. However, the prediction deviation remains below 8% throughout the entire service cycle of the liner plate. This model provides practical guidance for optimizing the use of intermediate roll bearing seat liner plate in cold tandem rolling mills.
To improve the low-temperature impact toughness of thick gauge offshore steel and achieve an excellent strength-toughness balance, the rolling and water cooling processes of 80 mm thick ness EH36 plates produced by a factory were studied. By regulating the microstructure through seven different processing schemes, the aim was to improve the strength-toughness match and low temperature impact toughness. Tensile tests, impact tests, and metallographic observations were conducted on the trial plates. The results indicate that with a low-carbon microalloyed chemical composition, by ensuring at least two passes with a reduction rate exceeding 10% during the roughing rolling stage, an intermediate slab thickness of over2h (where h is the thickness of the finished steel plate), controlling the finishing rolling temperature below 800 ℃, and the final cooling temperature below 300 ℃, a refined acicular ferrite and bainite microstructure can be obtained, resulting in a good strength-toughness match of plates. Further optimization was achieved by introducing a relaxation treatment before water quenching to 300 ℃, which promotes precipitation of proeutectoid ferrite and enhances microstructure uniformity. This dual-process strategy significantly improved the impact toughness of the plate.
In order to reduce production costs and resource consumption, a low cost and high-performance 40 mm thick Q690D plate was developed using w(C)≤0.16% and Ti-Cr-B chemical composition design, coupled with clean steel production technology, controlled rolling under high reduction and DQ+T process. The plate was subjected to oblique Y-groove cold cracking sensitivity test and multi pass gas metal arc welding (GMAW) test, and the microstructure transformation characteristics and mechanical properties of the welded joint were analyzed. The results show that the yield strength of Q690D plate is 799 MPa, the tensile strength is 868 MPa, the elongation is 18.5%, and the impact energy at -20 ℃ is 186 J, which is sufficient margin compared to the standard. However, the welding heat affected zone has a certain tendency to harden. According to the cold cracking sensitivity welding test at -5 ℃, the tested plate has good cold crack resistance after welding without preheating and with a heat input of 15.8 kJ/cm. After multi pass GMAW welding with ER80-G solid wire with equal strength matching, the weld seam strength is 2.3% lower than that of the base metal, and the strength loss rate is small. The heat affected zone is mainly composed of lath bainite, granular bainite, proeutectoid ferrite and carbide. The impact energy of fusion line and coarse grain zone has a small decrease compared to the base metal, and the fracture type is ductile fracture.
To develop a new generation of high-performance bridge steel, the microstructure and mechanical properties of Q420qE bridge steel plates under different self-tempering temperatures during the cooling process after rolling were studied by using an optical microscope, a scanning electron microscope, tensile and impact tests. The results show that when the self-tempering temperature is between 610 ℃ and 670 ℃, as the self-tempering temperature increases, the yield strength and tensile strength of Q420qE steel continuously decrease, the elongation after fracture continuously increases, and the impact energy at -40 ℃ and the yield ratio first increase and then slightly decrease. When the self-tempering temperature drops from 670 ℃ to 610 ℃, the transformation sequence of the matrix microstructure of Q420qE steel is as follows:ferrite + pearlite → granular bainite + ferrite + a small amount of pearlite → feathery bainite + granular bainite + ferrite + a small amount of pearlite, when the microstructure is uniform and fine ferrite + pearlite, the impact toughness is the best, but the strength is relatively lower. When the microstructure is feathery bainite, the strength is the highest, but the impact toughness deteriorates. When the self-tempering temperature is controlled between 630 ℃ and 650 ℃, the 30 mm thick Q420qE bridge steel plate obtains a mixed microstructure of granular bainite + ferrite + a small amount of pearlite, which meets the requirements of the new generation of high-performance bridge steel with high strength, good low-temperature toughness and a low yield ratio.
In order to improve the properties of transformation induced plasticity steel, the effects of isothermal treatment on the microstructure and mechanical properties of transformation induced plasticity (TRIP) steel were investigated through annealing simulation experiments. The results indicate that when TRIP steel undergoes isothermal treatment at 300-370 ℃ (below Ms), its tensile strength and yield strength decrease, while elongation increases with the increase of isothermal temperature. However, at 420 ℃ (above Ms), the TRIP steel exhibits lower yield strength and elongation. Compared to the isothermal treatment at 420 ℃, the microstructure of TRIP steel treated at 300-370 ℃ is relatively finer, and the microstructure consists of lath martensite, bainite and retained austenite. At 370 ℃, TRIP steel demonstrates a higher volume fraction of bainite and retained austenite, resulting in superior mechanical properties.
Low-carbon steel bars are highly susceptible to Widmannstatten structure during the rolling process, resulting in properties degradation. Using 20 steel as the research subject, the phase transformation temperature as well as the recrystallization completion temperature of 20 steel were obtained by thermodynamic and theoretical calculations. The effect of different heating temperatures and holding times on the microstructure of 20 steel billets was studied in the laboratory. Combined with finite element simulation software, the heating system of billets was optimized and industrial trial production was carried out. The results show that billets heating temperature is the most important factor affecting the Widmannstatten structure, followed by the holding time. The improved heating regime features: first heating zone temperature was (920±10)℃, second heating zone temperature was (1 050±10)℃, soaking zone temperature was (1 050±10)℃, with total furnace residence time of 120 min. This enables low-temperature rolling of 20 steel bars in mass production, with initial rolling temperature at 950 ℃ and final rolling temperature at 860 ℃. The average grain size decreased from 40 μm before improvement to 26 μm after optimization, while effectively suppressing Widmanstätten structure formation. Under the synergistic effect of grain refinement and microstructure homogeneity, the mechanical properties of 20 steel bars are greatly improved, the yield strength is increased from 270 MPa to more than 300 MPa, and the reduction of area is increased from 50% to more than 60%.
In view of the transverse stripe defects appeared on the surface of SR04 strip after electrocoating under the 2C1B intercoat-free process, its surface morphology, mechanical properties and microstructure were analyzed, and comparisons with those of the production lots without that defect were carried out. The reasons for the occurrence of transverse stripe defects were identified, and targeted improvement measures were proposed. The results show that the root causes of the transverse stripe defect after coating are that the cold rolled SR04 strip has non-smooth surface morphology and inhomogeneous size of ferrite grain. Therefore, optimization measures of chemical composition design and whole processing were put forward. By increasing the boron content and decreasing the carbon content, optimizing the hot rolling lubrication process comprehensively, decreasing the coiling temperature, optimizing the surface morphology of work rolls of acid pickling, cold rolling and leveling, decreasing the soaking temperature, decreasing the roll roughness and the elongation of leveling, the surface morphology of cold rolled SR04 strip and the uniformity of ferrite grain size has been significantly improved. After the optimized processes were carried out, the problem of transverse stripe defect under intercoat-free process was solved. The comprehensive quality of the optimized SR04 strip products has reached an excellent level of steels for similar intercoat-free outer panel in foreign steel works.
To address the frequent defects such as zinc flow marks, zinc dross, coating pits, color variation, overly large spangle contours, and poor fineness and uniformity of the coating, low product qualification rate during the initial development of hot-dip galvanized sheets for IT-grade at Panzhihua Steel, optimization and improvements to the production process were required. The equipment and process characteristics of Panzhihua Steel′s hot-dip galvanizing line were introduced. The substrate surface quality, zinc bath composition design, furnace nose process, zinc pot parameters, and skin-pass process were analyzed and optimized. The production results indicate that by controlling the substrate surface roughness within 0.8-1.8 μm, maintaining the Al mass fraction in the zinc bath at 0.21%-0.23%, regulating the zinc bath temperature to (460±2)℃, setting the strip temperature into the zinc pot at 470-490 ℃, adjusting the air knife pressure to 0.02-0.03 MPa, and implementing high rolling force with a high-concentration wet skin-pass process, the qualification rate of galvanized sheets for IT-grade increased from 64.54% to 99.72%, enabling stable mass production.
In view of the problem of long natural aging time in the production of high carbon steel SWRH82B wire rod in winter, the relationship between the reduction of area of SWRH82B wire rod and natural aging time was investigated. The results show that the reduction of area of high carbon steel SWRH82B wire rod is mainly affected by its hydrogen content. With the extension of aging time, the hydrogen mass fraction continues to decrease, and the reduction of area gradually increases. When the temperature is between -25 ℃ and -20 ℃, the wire rod is aged for 15-17 d and the hydrogen mass fraction is reduced to 0.24 × 10-6, the reduction of area increases to 28%. With extension of aging time, the change of hydrogen mass fraction and reduction of area tends to stabilize. Improving the aging temperature and extending the aging time can reduce hydrogen content in the wire rod. In this regard, the cooling process of SWRH82B wire rod is optimized, and the roller table speed is controlled from 0.70 m/s to 0.80 m/s, the opening of fan No.1-No.3 is 100%, the opening of fan No.4-No.6 is 90%, the opening of fan No.7-No.10 is 70%, and the fans at other positions are closed, and the wire rod is slowly cooled by the residual heat after phase transformation, which realizes online aging, and the aging time can be shortened to 4 d, which improves the production efficiency.
