,

1)Hot Rolling Plant,Baoshan Iron & Steel Co.,Ltd.,Shanghai 200941,China;2)Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201999,China

Abstract: Ultra-thin hot-rolled strip is the developing result of the modern plate and strip rolling technology and can be used to replace cold-rolled products for energy saving and emission reduction.This work reviews recent progress and breakthroughs in this field by focusing on three recent technologies,namely the conventional strip with hot coil box and endless rolling,thin slab continuous casting and rolling/semi endless rolling,and endless compact rolling.In summary,ensuring high-quality production is an important bottleneck for ultra-thin strip and ferritic hot rolling by endless compact rolling.This production technology has strong feasibility and broad prospects for future development.

Key words: hot rolling; ultra-thin; compact production technologies; endless rolling; ferritic rolling

1 Introduction

Ultra-thin strip refers to a strip with more than 700 mm width and less than 1.6 mm thickness and is basically produced by cold rolling in the past.With advances in conventional hot strip rolling mill (HSM) and thin slab continuous casting and rolling (TSCR),the production of ultra-thin hot-rolled strip has increased,and the thickness has gradually dec-reased.As a result,hot-rolled products have become a viable alternative for cold-rolled ones,i.e.,“hot instead of cold”.A few years ago,it was estimated that approximately 25% of cold-rolled sheets could be replaced by thin gauge hot-rolled strip[1];on the basis of this estimation,the annual demand for thin gauge “hot instead of cold” strip in China is approximately 15 Mt[2].Considering the develop-ment of steel increment and “high strength thin-ning”,the annual growth rate of “hot instead of cold” strip (approximately 60% of the cold-rolled strip of low carbon nonalloy steel with a thickness be-tween 0.7 mm and 1.2 mm[3]),and the improve-ment of quality and properties of hot-rolled strip steel,the demand for “hot instead of cold” appears to be larger than this estimated value.

In addition to its large market capacity,“hot instead of cold” is in line with the development trend of energy saving and emission reduction,green technology,and low-cost manufacturing.Therefore,ultra-thin hot strip rolling has become the focus of hot strip rolling practitioners.This work analyzed and compared existing equipment and technologies for ultra-thin hot-rolled strip.The results serve as a reference for relevant professionals to further deve-lop and improve the technology and quality of ultra-thin hot-rolled strip.

2 Present situation of ultra-thin hot strip rolling equipment technology

Ultra-thin strip rolling is usually conducted at high temperature and large deformation with fast strip velocity and violent mill vibration by using high rolling force and big temperature drop.The process often causes accidents such as roll chock,strip end flip,and unstable running on the run-out table.Defects such as waviness,tower-shaped coil,and pockmarks also easily occur.Therefore,improv-ing the rolling stability is the primary task for ultra-thin gauge rolling.In addition to functional accuracy and control level of finishing mill equipment,roll-ing temperature uniformity is important.When the temperature of the rolled strip is low,a large number of scrap,steel sticking,and tail flick might be generated due to the increase in deformation resis-tance.When the transverse and longitudinal temper-ature uniformity of strip is poor,the rolling process would be unstable and prone to strip breakage[4].

Different hot rolling equipment and technologies have inherent variations in temperature uniformity.For the conventional HSM,the transverse temper-ature difference is large due to the long duration for multi-pass rough rolling and descaling.Owing to the beam of reheating furnace and the difference in waiting time for the transfer bar to finish rolling,the longitudinal temperature difference is also enlarged.For thin rolling,the temperature difference must be improved to a certain extent by applying a hot coil box (CB).For TSCR,the slab enters the finishing rolling stage in a short time after leaving the tunnel furnace.Here the longitudinal temperature is uniform,and the transverse temperature difference is greatly improved.For endless strip production (ESP),despite the large edge temperature drop after the slab has been dischar-ged from the caster,the stability of direct rolling can be realized due to the large gap of roughing roll.How-ever,for high alloy steel,direct roughing will produce edge defects.The transverse temperature difference is reduced by induction heating before finishing rolling,and the uniform length temperature ensures that the ultra-thin specification can be rolled stably in finishing rolling.Thin strip casting and rolling is mostly single stand rolling with a small total reduction.This process can avoid the negative effects of large transverse temperature differences on rolling stability.

2.1 HSM

In the production of hot-rolled ultra-thin strip,the heat dissipation of rolled piece is fast,and the temperature is not easy to control.In addition,the high strip rolling speed makes the threading unstable and prone to steel sticking.Owing to the large rolling load,the rolling stability is easily damaged due to the small change of the rolled piece or the small disturbance of the rolling mill itself.Controlling the thickness and shape becomes dif-ficult due to untimely regulation.Therefore,corre-sponding technical measures must be taken to ensure the smooth production of thin strips.The CB and endless rolling technology can be adopted in traditional production lines to solve these problems.

(1) HSM

Conventional hot strip rolling mainly refers to the hot strip rolling line using a slab of more than 200 mm thick.In the past,3/4 roughing mill was used,but the double reversing roughing mill with seven finishing mills is widely chosen nowadays.Manu-facturing a thin product necessitates high rolling requirements on the equipment operation level,con-trol level,and operation technology.Baosteel 2 050 mm hot rolling has maintained high equipment func-tion precision and automatic control level for more than 30 years.Technicians and operators have accu-mulated rich experience in stable rolling.Mass produc-tion of 1.46 mm-thick high strength (yield strength 500 MPa) high surface pickling strip and 1.5 mm-thick QStE700TM was achieved.The latter has been used in bus frames to replace cold-rolled products since 2012.The rolling proportion of 2 050 mm hot rolling with high strength and ultra-thin (yield streng-th≥400 MPa and rolling thickness≤1.5 mm) is more than 1.2%.The proportion of POSCO hot-rolled ultra-thin plates is relatively high,especially due to the app-lication of endless rolling technology.The thickness below 1.6 mm of POSCO No.2 hot rolling accounts for more than 3%.

(2) HSM+CB

The main function of the CB is to coil the transfer bar after roughing mill,decrease the temperature drop of the bar and the temperature difference between the head and tail of the bar,and improve the rolling stability of the thin gauge and high defor-mation resistance varieties.

The use of a CB can improve the transverse and longitudinal temperature uniformity of the interme-diate billet and realize constant speed rolling.This process helps stabilize the rolling process.With the new generation of CBs,the temperature difference between the head and tail of the bar is reduced from 70-120 K to approximately 20 K,the transverse temperature difference is reduced from 100 K to less than 30 K,and the temperature difference between the upper and lower surface is reduced from 40 K to approximately 10 K.These values can meet the strict requirements of bar temperature uniformity for stable rolling of ultra-thin and high strength limit gauge strip in finishing rolling and prevent finishing rolling accidents.In addition,the use of a CB can reduce unplanned roll change,improve the uniformity of finish rolling temperature and coiling temperature,and reduce the furnace temperature or finish rolling power consumption to achieve energy saving and emission reduction[5].

Using a CB,Baosteel 1 422 mm hot rolling can stably roll 1.2-2.0 mm ultra-thin strips in large quantities,and the proportion of specification less than 1.6 mm is the highest among the nine conven-tional hot rolling lines of Baosteel[6].Chenggang of HBIS Group Co.,Ltd.has successfully developed 700L hot-rolled strip with a 1.2 mm thickness;the product has good shape and can be placed on the market after stable production of 500 t hot-rolled strips[6].Xichang of Panzhihua Iron and Steel Co.,Ltd.has realized the supply capacity of less than 1.6 mm pickling plate by manufacturing more than 1 000 t per month.The new CB for Baosteel’s 1 580 mm hot rolling mill had been put into opera-tion in 2021.

(3) Endless rolling of HSM

The endless rolling technology of HSM is that the bar after rough rolling is welded with the tail of the previous strip before entering the finishing mill.Continuous finish rolling is then facilitated.Endless rolling can maintain constant tension of the whole coil,realize stable rolling,and avoid the change of strip thickness and crown caused by prediction error and adjustment error from thermal roll expansion and wear model.As a result,the accuracy of plate thickness and shape is enhanced[7],and the specifi-cation range of conventional hot strip mills is expanded.When endless rolling is adopted,the strip with a thickness of 1.6 mm can be rolled to 1 600 mm width,and the strip with a width less than 1 250 mm can be rolled to 0.8 mm thickness[8].

This technology was first developed by the Kawasaki Chiba works in March 1996 on the newly built 3#2 050 mm hot strip mill[9];its endless rolling ratio was close to 1/3.A hot-rolled strip with a thickness of 0.8 mm was produced in August 1996[10].In 1998,Nippon Steel began to apply this technology in Oita works[11].In 2007,POSCO of South Korea adopted a new super-deformation join-ing technology (shear and pressing) in its No.2 hot strip production line to realize the mass production of endless hot strip rolling.Combined with dynamic thickening,this technology can realize the rolling of 10-20 pieces with a thickness range of 1.1-1.3 mm[12].All products with thickness less than 1.6 mm can be rolled using this technique,and the end-less continuous rolling of approximately 2.3 mm can be adopted for up to 90 pieces.

2.2 TSCR

TSCR is a new technology successfully developed in the late 1980s for the production of the hot-rolled coil.This approach mainly includes six types,namely,compact strip production (CSP) of SMS,inline strip production (ISP) of DEMAG,quality slab pro-duction (QSP) of Sumitomo,flexible thin slab rolling (FTSR) of Danieli,continue rolling (CON-ROLL) of VAI,and thin slab production (TSP) of Tippins in the United States[13].ESP and other endless rolling equipment of compact process are also considered as a new generation of TSCR technology.

The main difference between TSCR and HSM lies in the production of ultra-thin products.Lianyuan Iron and Steel Co.,Ltd.(Lianyuan),Tangshan Iron and Steel Co.,Ltd.(Tangshan),and Guangzhou Zhujiang Steel Co.,Ltd.(Zhujiang) produced 0.78-0.97 mm-thick hot-rolled coils[14].From January to September 2006,1.299 Mt of hot-rolled coils with thickness≤2 mm were manufactured in 12 TSCR production lines in China;this value accounted for 7.1% of total production.For the products with this thickness,the CSP of Zhujiang has reached 53.8%,and Tonggang and Masteel have reached 16.0% and 13.0%,respectively.The ISP of the Italian Arvidi plant kept the production of hot-rolled coils with thickness≤1.5 mm using a 65-mm-thick slab for a long time,and the production capacity of a single flow caster was more than 1.2 Mt/a.Rizhao ESP trial was able to roll 0.6 mm-thick hot-rolled strip.

(1) Coil-to-coil rolling of TSCR

Owing to the close distance between the mill inlet and the furnace outlet,the short temperature drop time results in good temperature uniformity that is suitable for the production of ultra-thin products.Therefore,ultra-thin rolling is an important develop-ment trend of TSCR[15].With CSP as an example,the slab temperature difference can be kept within ±10 K even if a single slab is rolled[16].

The CSP production line of Wuhan Iron and Steel Co.,Ltd.of Baosteel (Wuhan) took advantage of seven finishing stands to successfully produce the world’s first 0.8 mm-thick hot-rolled coils by single rolling in 2017.This line has previously realized the batch application of 1.20 mm-thick 700 MPa grade products.

In 2009,the Zhujiang CSP line developed the pro-duction technology of ultra-thin hot-rolled strip.In the same year,the output of ultra-thin 1.00-1.35 mm hot-rolled strip was 30.3 kt.The proportion of products with 1.35 mm and below reached 8.0%,and that of products with 1.50 mm and below reached more than 50%.Among them,the mini-mum thickness of ordinary carbon steel in batch pro-duction is 1.20 mm,that of container plate is 1.40 mm,that of pattern plate is 1.80 mm,that of high strength steel plate with a yield strength of 550 MPa is 1.50 mm,and that of high strength steel plate with a yield strength of 700 MPa is 2.0 mm[16].

(2) Semi endless rolling of TSCR

Semi endless rolling refers to the process adopted by the TSCR production line to manufacture a slab with a maximum length of 4-6 times the ordinary slab to realize continuous rolling.The flying shear in front of the coiler cuts it into the required quality coil.The ultra-thin strip is produced by changing the thickness of the strip in the continuous semi endless rolling process.The effective thickness change and stable rolling are achieved by controlling the roll gap and the speed of each mill.As a result,the length of non-uniform thickness is shortened,and the strip tension is adjusted to the target value[15].The three TSCR production lines of Masteel,Tangshan and Lianyuan are equipped with “semi endless roll-ing” equipment,which is reserved by CSP of Wuhan.

The CSP of Lianyuan has created a record of continuously rolling 269 m ultra-long continuous casting slab into seven coils with a minimum strip thickness of 0.77 mm through multiple debugging and process optimization.In the semi endless rolling products,the proportion of thin products less than 2.0 mm is more than 96.3%,that of one hot-rolled roll divided into more than three is more than 92%.The thickness deviation of more than 92% semi endless rolling strip with a total length of more than 5 500 m is stable within 15 μm,and the thickness accuracy of the strip is remarkably improved[17].

The FTSR of Tangshan uses the function of FGC and the equipment of PC angle dynamic adjustment under load state to ensure the product quality in the production of ultra-thin hot strips[18].The propor-tion of products with 1.5 mm and below specifi-cations can reach 70%-75%,thus realizing the mass production of 1.0 mm ultra-thin hot-rolled strips[19].

2.3 Endless rolling technology of compact pro-cess

ESP is a new thin slab continuous casting and rolling technology that mainly realizes “one (continuous casting) flow to one (rolling) mill” by high casting speed.The continuous and stable casting and rolling of the strip are accomplished using a series of equipment such as transfer bar heat,dynamic roll shifting,and flatness measure-ment system with tension.Through thick specifi-cation (3.0-4.5 mm) threading,the rolling thick-ness can be reduced to 0.8 mm or even 0.6 mm.In 2009,the ESP line of Avidi Company in Italy was built and put into operation.The proportion of pro-ducts with thickness less than or equal to 1.5 mm exceeded 21%,and the thinnest strip was 0.8 mm×1 540 mm[20].Other companies that can realize all endless rolling in compact casting and rolling include the CEM of POSCO and DUE of Danieli.

Rizhao has successively put into operation five ESP production lines from 2015 to 2020.In May 2019,the Shougang Jingtang MCCR production line was put into operation.Quanfeng Sheet’s “energy-saving ESP” has also been put into operation.Several ESP projects in China and abroad are currently under construction.

ESP roughing mill is equipped with long stroke AGC hydraulic cylinder,positive bending and nega-tive bending system,dynamic cooling of work roll,tension roll,and inspection equipment for thick-ness,crown,wedge,and width,which ensures good strip profile after roughing.The crown/wedge/flatness detection can realize the accurate control of the profile of the transfer bar and provide a good incoming shape for the finishing rolling.With the improvement of temperature uniformity,the foundation of ultra-thin rolling is finally formed.

The temperature and the temperature uniformity in transverse and longitudinal directions are improved by the heat supplement device at the finishing ent-rance,which further provides conditions for the stability of ultra-thin finishing rolling.The five stands of finishing mill are equipped with long stroke hydraulic AGC,positive bending roll system of work roll,dynamic roll shifting system with load,dynamic cooling system of work roll,tapered roll,low inertia quick response looper and rolling lubrication system,surface detection system,contact flatness measuring roll,and rolling lubrication to ensure the high thickness precision,good shape,and rolling stability of ultra-thin specification.

Compared with conventional hot rolling and TSCR,the main and core progress of ESP-like equipment is to realize the continuity and stability of the finishing process,even the change factor of thickness can be gradually modified to ensure rolling stability.Therefore,ESP-like technology can stably roll wide strip below 1.5 mm in batches,and the thickness of 0.8-1.5 mm is the most cost-effective and competitive specification.With Rizhao ESP as an example,the single-pass rolling kilo-meter can reach more than 224 km,the average rolling kilometer is 162 km,the maximum number of pouring heats is 13 heats (nominal furnace capacity 300 t/heat),and the average thickness specification proportion of ≤1.2 mm is 40.1%.The proportion of specification ≤0.8 mm,≤1.2 mm,and ≤1.5 mm can reach 13.8%,65.7%,and 80.2%,respectively.These products and specifications can truly achieve a wide range of “hot instead of cold” .

2.4 Strip casting

Twin-roll strip casting and rolling is a technology that uses liquid metal as raw material and two casting rolls with opposite rotation directions as crystallizers to produce strip directly from the liquid metal.Since the 1990s,many iron and steel com-panies have focused on the production of twin-roll casting strips.Castrip of Nucor is the first produc-tion line of twin-roll ultra-thin casting strip (UCS) in the world.After being added into production in 2002,0.9-1.5 mm-thick plain carbon and low carbon thin strips have been generated,and the thickness range can be extended to conventional hot-rolled strip products.These materials can replace cold-rolled strip products.Some structural products can also be produced[21].The thinnest strip thickness by Castrip is 0.84 mm,and this material can directly replace the cold rolling product[22].

The first strip casting and rolling industrial pro-duction line independently integrated by Baosteel in China was put into trial operation in 2014.This line is the fifth industrial production line in the world,following Nucor of the United States,Thyssen of Germany,Nippon Steel of Japan,and POSCO of Korea.In 2015,the line produced ultra-thin con-tainer steel and other products and successfully rolled 0.9 mm ultra-thin hot-rolled strip.

Thin strips of stainless steel,silicon steel,and high-speed steel with a thickness of 1.0-3.0 mm were cast and rolled in the laboratory or pilot demonstration line of Northeastern University[23].In March 2019,Shagang Group introduced Nucor’s Castrip equipment and technology to realize industrial production.The total length of the thin strip production line is designed to be 50 m,the thin plates of 1.4-2.1 mm are directly cast,and the target thickness of 0.7-1.9 mm is rolled by a rolling mill.The maximum width is 1 590 mm.

Owing to the advantages of rapid solidification of strip casting,this method is especially suitable for high alloy metal materials.From this perspective,strip casting and conventional processes are highly complementary.The materials (usually high alloy steel and nonferrous metal materials) that are dif-ficult or cannot be processed by conventional hot roll-ing are easy to segregate,precipitate large blocks,eutectic,and peritectic.As a result,the products have low plasticity and high deformation resistance and there-fore are suitable for thin strip continuous casting.

3 Ferritic rolling:a typical process for hot-rolled ultra-thin strip

Ferritic rolling indicates that the phase transformation (γ→α) is completed before the rolling piece enters the finishing mill.Hence,the finishing rolling process is completely carried out within the scope of ferrite.Roughing is still finished in austenite.Through the ultra-fast cooling system between the finishing mill and roughing mill,the strip temperature can be reduced to belowAr3prior to the finishing mill.

The original intention of ferritic rolling tech-nology is to realize ultra-thin products.Ferritic roll-ing is mainly used for low carbon steel,ultra-low carbon steel,or IF steel.Especially for ultra-low carbon steel,the conventional thickness of hot rolling is more than 2.3 mm to ensure high finish-ing rolling temperature and avoid the deterioration of rolling stability and mixed crystal structure in two-phase rolling.The successful development of ferritic rolling technology paved the way for the mass production of ultra-thin strips due to the low-temperature single-phase rolling.On the one hand,this process ensures the smooth production of the mill and will not reduce productivity.On the other hand,the ferrite zone rolled strip has good performance,and product quality can be basically guaranteed.At the end of the 1980s,Belgium Iron and Steel Research Center successfully researched and developed the ferritic low-temperature hot rolling and successfully applied this new process to the industrial production of ferritic rolling in the Cockerill Sambre steel plant in Belgium to produce ultra-thin (less than 1.0 mm) hot-rolled strip with good deep drawing performance as a replacement for some cold-rolled products[24].Baosteel Zhanjiang 2 250 mm hot rolling mill successfully and stably roll-ed 1.0 mm thick strip with good shape and dimen-sion quality by taking advantage of the characte-ristics of ferritic rolling,such as no phase transfor-mation and low threading speed.

The products of ferritic rolling include hot-rolled strip,general cold-rolled steel,deep drawing or ultra-deep drawing cold-rolled steel,and directly annealed steel plate after rolling in ferrite zone.The directly app-lied hot-rolled strip is rolled in the high-temperature range of the ferrite region,and the fully recrystallized ferrite is then obtained by high-temperature coiling.However,the directly annealed steel plate,after hot rolling,is rolled in the low-temperature range of the ferrite region.Recrystallization cannot be completed in subsequent coiling.Hence,subsequent heating is needed to improve the microstructure and proper-ties.

Lubrication rolling technology is the key tech-nology to realize ferritic rolling.If lubrication roll-ing is not used,then the friction between strip and roll will cause a large amount of shear texture (110)[uvw] in ferrite on the surface of the strip that will affect its deep drawing performance.After cold rolling and annealing,thervalue of the hot-rolled strip is extremely high,with an average of 3.0[25].However,this result has not been further verified.

4 Analysis on hot rolling technology of ultra-thin strip

The typical production lines of four types of hot rolling equipment for ultra-thin strip production are summarized in Table 1.

Table 1 Rolling status of ultra-thin gauge for four kinds of hot rolling equipment

4.1 HSM

In the early stage,the technological characteristics of CB have not been comprehensively understood.In addition,the first generation of CB with mandrel encountered some problems,such as the temperature drop at the end of the middle coil that may lead to rolling instability and affect the production capacity of the rolling mill.These issues also affect the correct understanding and evaluation of CB and the use and promotion of CB in the hot strip production line in China[5].With further development of CB and the improvement of the maintenance level of the steel plant equipment,the application of this approach has broadened.The production line of this equipment has been incorporated into operation and planned to increase.After the application of CB,the problem of tail temperature drop is solved,constant speed rolling is realized,and the transverse temperature uniformity is improved.The products with a thickness of 1.2 mm can be rolled stably,but the process remains single billet rolling.The stability and dimensional accuracy of the strip during threading and throwing still failed to meet the requirements.

After the CB,the technology of online dynamical thickness change can be applied to further reduce the thickness and broaden the rolling specifications by setting the intermediate billet connector and high-speed slitting shear to realize endless rolling.However,restricted by the bar connection tech-nology,the existing three connection technologies (induction heating,laser welding,and solid-state pressing) cannot meet the requirements of stable production.For example,although the welding success rate with POSCO No.2 hot rolling can reach 99.9%,further improvement is still needed for the hot continuous rolling production line to achieve a high operation rate,low accident rate,and low cost.In addition,the different temperature uniformity near the welding joints might further lead to the instability of the finishing process.This bottleneck hinders the large-scale popularization of conventional hot rolling endless rolling.

Even without considering the joint problem,the temperature uniformity of the bar is not as good as that of the ESP-like equipment.At present,the traditional ultra-thin production line can only utilize the characteristics of high reduction rate,long-term oxidation,and multiple descaling to seek high-end application products from the performance and surface.Complementing with ESP-like products in variety,quality,and rolling cost remains necessary.Therefore,HSM and ESP-like lines will coexist for a long time.

4.2 TSCR

The starting finishing rolling temperature of TSCR is generally controlled at 1 100-1 150 ℃,which is 100-150 K higher than that of HSM.Therefore,the former is more suitable for ultra-thin strip rolling than the latter.However,temperature-caused and roll-caused scales easily occur due to the high temperature and slow rolling speed of the front stand of the finishing mill.In addition,the FeO scale is difficult to be removed without further oxidation due to the short time allotted for reheating and finishing rolling immediately after heating.Other problems include strip surface defects,such as the scratch of the furnace roll and the heavy load of the front stand[26].

Semi endless rolling of thin and ultra-thin products has been widely used for replacing cooling with heat.This approach advantages over single bar rolling in terms of profile quality and expanding the range of thin strip specifications.However,for the TSCR production line,one of the continuous casters and the A tunnel furnace are on the same centerline as the rolling mill.Thus,this continuous caster can produce long slabs with semi endless rolling.Another continuous caster with the B tunnel furnace can only pour short slabs at a slow speed,thereby leading to single slab rolling or even stoppage of casting.Therefore,the semi endless mode will inevitably lead to declined production capacity,increased cost,and reduced product competi-tiveness.

4.3 Endless strip rolling of compact process

The ESP-like production line is the most suitable technology for the “hot instead of cold” principle because of its complete endlessness and stability.Its strong competitiveness dominates the 0.8-1.5 mm cold rolling applications and impacts the 1.5-2.0 mm conventional hot strip rolling market.This approach poses a great threat to traditional TSCR technology for ultra-thin hot strip.

With technology development and equipment improvement,the surface quality defects and low elongation of ESP-like products must also be improved continuously.In addition to molten steel quality,composition design and steel grade must be enhanced according to equipment characteristics to realize “endless rolling+compact production+high quality” and greatly broaden the ultra-thin steel grade and application range of “hot instead of cold”.

(1) Technological innovation.By “online roll change”,ESP has begun to try to further extend endless rolling kilometer,improve surface quality,reduce cost,and improve the working ratio.

(2) Variety adaptability development.With the continuous and stable operation of the laminar cooling roller table,we can try to add heat treatment equipment after finishing rolling and before coiling so as to improve the quality and expand the variety and carry out ferritic rolling.More refined equipment with pressure jet cooling can also be developed.

(3) Ferritic rolling.Ferritic rolling can be accom-plished via three paths.① High-temperature ferritic rolling+high-temperature coiling:direct applica-tion,suitable for ultra-thin products with “hot instead of cold”.② Low-temperature ferritic rolling+annealing and omitting cold rolling:suitable for ultra-thin rolling.③ Low-temperature ferritic rolling+annealing+cold rolling:the subsequent cold rolling and annealing is the same as that of conventional rolling,but the process parameters must be chang-ed.

The first path is the best because of its energy-saving feature,which is the goal of the equipment,process,and technology.The ESP-like production line exhibits this characteristic and thus is suitable for ferritic rolling in the first path.

(1) Ultra-low carbon steel and low carbon steel that meet the market demand are the most suitable for ferritic rolling,and low carbon steel is the most suitable variety for ESP,which can be produced at high casting speed so as to improve the production capacity and further reduce the cost.

(2) ESP-like process has uniform transverse temperature,which further improves the uniformity of properties between strip edge and center.

(3) The distance between the finishing mill and coiler is shortened,which can coil ultra-thin strip at high temperature and realize recrystallization of the ferritic rolled strip.

(4) Induction heating can be avoided in ferritic rolling to prevent the temperature scale produced by high temperature rolling with supplementary heat-ing.In this case,the inlet temperature of the finishing mill is still surplus,and a high-pressure descaling may be added for thorough secondary scale removal.

ESP uses ferritic rolling technology to produce more than 100 000 t of products every month,thus greatly reducing the power consumption of supplemental heating prior to finishing rolling.However,ESP has only one descaling before finishing rolling,and the descaling effect of strip steel is relatively poor at lower temperatures.Therefore,proper heat supplement must still be performed before finishing rolling and cannot be completely omitted.After induction heating,the entrance temperature of finishing rolling is high,and the front stand is rolled in the austenite and two-phase zones.Hence,the stamping performance of the product does not show advantages over those produced through conventional rolling.Therefore,the surface quality of the strip,particularly that of the casting slab,must be solved first to fully realize the natural advantages of “endless rolling+compact production+ferritic rolling” technology.

“Endless rolling+compact production+ferritic rolling” is an excellent combination of equipment and technology.If the surface defects can be addressed,then the cost,quality,performance,and other aspects of the materials will be greatly improved compared with those of current ESP-like products.This approach will expand the application scope of “hot instead of cold” and enhance the competitiveness of ESP-like technology.

5 Conclusions

(1) Product quality is affected by the instability of the head and tail of conventional hot strip rolling and TSCR in ultra-thin billet rolling,which is not suitable for ultra-thin production.Semi endless and endless rolling are conducive to thin rolling.

(2) The connection technology of endless rolling in conventional hot strip rolling is still uncertain.The application of a CB can greatly improve the production capacity of the traditional production line.

(3) TSCR semi endless rolling affects the pro-duction capacity and results in the lack of cost competitiveness.Its possible advantage lies in its thin gauge;hence,the effect of ESP-like technology on TSCR semi endless rolling is greater than that on conventional hot strip rolling.

(4) ESP-like technology is the best equipment for the ultra-thin specification of “hot instead of cold”.Steel grade development and quality impro-vement according to the equipment characteristics must be achieved to realize “endless rolling+compact production+high quality”.

(5) The combination of “endless rolling+com-pact production+ferritic rolling” can further im-prove the quality of ultra-thin products and reduce the production cost.Breakthroughs using this app-roach are currently lacking and are therefore worthy of future efforts.