Deformation Behavior and Experiments on a Light Alloy Seamless Tube Via a Tandem Skew Rolling Process

Article Deformation Behavior and Experiments on a Light Alloy Seamless Tube via a Tandem Skew Rolling Process

Feilong Mao 1,* , Fujie Wang 1, Yuanhua Shuang 2, Jianhua Hu 2 and Jianxun Chen 2

1 Department of Mechanical and Electronic, Yuncheng University, Yuncheng 044000, China; [email protected] 2 The Coordinative Innovation Center of Taiyuan Heavy Machinery Equipment, Taiyuan University of Science and Technology, Taiyuan 030024, China; [email protected] (Y.S.); [email protected] (J.H.); [email protected] (J.C.) * Correspondence: [email protected]; Tel.: +86-359-859-4433

Received: 29 November 2019; Accepted: 25 December 2019; Published: 29 December 2019

Abstract: As a process for producing seamless tubes, the tandem skew rolling (TSR) process was proposed. In order to study deformation characteristics and mechanism on tubes obtained by the TSR process, a numerical simulation of the process was analyzed using Deform-3D software. Simulation results demonstrated the distribution of stress, strain, velocity, and temperature of a seamless tube in the stable stage during the TSR process. Actual experiments of carbon steel 1045, high strength steel 42CrMo, and magnesium alloy AZ31 were carried out in a TSR testing mill. The results demonstrated that the TSR process is qualiﬁed for producing tubes of high quality, with an accuracy of 0.2 mm in ± wall thickness and 0.35 mm in diameter. This process is suitable for manufacturing seamless tubes ± that are diﬃcult to deform or that have been deformed in a narrow range of temperature.

Keywords: tandem skew rolling; seamless tube; magnesium alloy; deformation behavior; high strength steel

1. Introduction The steel industry is now in a post-industrial era of steel, whose theme is “green manufacture, make green.” The short process is one way to meet the requirement for industrial development. The tandem skew rolling (TSR) process is an ingenious metal-formed technique for producing seamless steel tubes that can achieve continuous skew rolling by combining the piercing section with the rolling section [1,2]. Compared with the conventional process of manufacturing seamless steel tubes, a three-step forming process of piercing, rolling, and reducing, the TSR process is a shorter, two-step forming process. The TSR process has been investigated for many years. In order to implement the TSR process, a TSR testing mill was designed and developed for experimental research. The mechanical structures of the mill and a method of adjusting process parameters were introduced and described [3,4]. A theoretic model of the TSR process was then established and derived in terms of dual stream functions, which could obtain the kinematically admissible velocity field of the billet during the rolling process [5]. Based on the times spent on the deformation process of the two sections, the velocity coordination of the rollers in both sections was researched, and a proper velocity match was obtained [6]. Moreover, to study the interstand tension in the tandem process, a tension testing device was developed and used with the testing mill, which reflected the changes in the stress of billet [7]. Further, how the process parameters affected the rolling force and the dimensional precision of the tubes was investigated [8]. Finally,

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Metals 2020, 10, x FOR PEER REVIEW 2 of 11 aiming to improve the dynamic performance of the control system in the TSR process, a dynamic the control system in the TSR process, a dynamic matrix control algorithm together with a particle matrix control algorithm together with a particle swarm optimization algorithm was used to optimize swarm optimization algorithm was used to optimize the parameters in the process [9,10]. the parameters in the process [9,10]. Due to the application of a finite element model (FEM), it was possible and economical to Due to the application of a finite element model (FEM), it was possible and economical to precisely precisely analyze the changes in the shape of workpieces during metal deformation. Wang et al. [11] analyze the changes in the shape of workpieces during metal deformation. Wang et al. [11] used the used the finite element method to analyze the three-roll skew piercing process. Bogatov et al. [12] finite element method to analyze the three-roll skew piercing process. Bogatov et al. [12] studied the studied the rotary piercing process using Deform-3D software and obtained the trajectories of points rotary piercing process using Deform-3D software and obtained the trajectories of points with different with different radial coordinates. Pater et al. [13] studied skew rolling with three tapered rolls for radial coordinates. Pater et al. [13] studied skew rolling with three tapered rolls for producing long main producing long main shafts of steel using the commercial FEM simulation software suite Simufact shafts of steel using the commercial FEM simulation software suite Simufact Forming v.12. They also Forming v.12. They also studied the tube forming process in Diescher’s mill [14], and the studied the tube forming process in Diescher’s mill [14], and the thermo-mechanical piercing process thermo-mechanical piercing process using the commercial software MSC SuperForm 2005 [15]. using the commercial software MSC SuperForm 2005 [15]. Stefanik et al. [16] studied the three-high Stefanik et al. [16] studied the three-high skew process for producing aluminum bars using the Forge skew process for producing aluminum bars using the Forge 2011® software. Romantsev et al. [17] 2011® software. Romantsev et al. [17] studied the piercing process for a new four-high screw rolling studied the piercing process for a new four-high screw rolling mill without guides, which was designed mill without guides, which was designed by comparing existing piercing processes in screw rolling by comparing existing piercing processes in screw rolling mills. Toporov et al. [18] used finite element mills. Toporov et al. [18] used finite element modeling to improve the piercing operation on a modeling to improve the piercing operation on a piercing mill. Murillo-Marrodán et al. investigated piercing mill. Murillo-Marrodán et al. investigated the friction conditions between the rolls and the the friction conditions between the rolls and the workpiece of a hot skew roll piercing process with workpiece of a hot skew roll piercing process with three friction laws via the FORGE software [19]. three friction laws via the FORGE software [19]. Aiming at the precise representation of the deformation characteristics of billets during the TSR Aiming at the precise representation of the deformation characteristics of billets during the TSR process, numerical simulation of the TSR process using the software Deform-3D was employed to process, numerical simulation of the TSR process using the software Deform-3D was employed to study the distributions of stress and strain. Based on numerical analysis, experiments were carried study the distributions of stress and strain. Based on numerical analysis, experiments were carried out with carbon steel 1045, high-strength steel (HSS) 42CrMo, and magnesium alloy AZ31 on the out with carbon steel 1045, high-strength steel (HSS) 42CrMo, and magnesium alloy AZ31 on the TSR testing mill to verify the feasibility of the process. The TSR process was originally developed to TSR testing mill to verify the feasibility of the process. The TSR process was originally developed to manufacture seamless tubes that were difficult to deform (e.g., high strength steel 42CrMo) or manufacture seamless tubes that were difficult to deform (e.g., high strength steel 42CrMo) or could onlycould be only deformed be deformed with a narrow with temperaturea narrow temperature range (e.g., AZ31range alloy) (e.g., because AZ31 thealloy) temperature because dropthe wastemperature slower in drop this process. was slower in this process.

2. Materials and Methods By the TSR process of producingproducing seamless tubes, the heated cylindrical billet was pierced and rolled continuously in the mill, in which the pierci piercingng and rolling processes were completed at at once. once. The schema of thethe TSRTSR processprocess isis presentedpresented inin FigureFigure1 .1. TheThe processprocess involvesinvolves threethree MannesmannMannesmann barrel-shaped rollers in the piercing section, three Assel tapered rollers in the rolling section, and a plug together with a mandrel andand aa billet.billet.

Figure 1. Schema of the tandem skew rolling (TSR) process. 2.1. Experimental Test 2.1. Experimental Test Through untiring eﬀorts made over many years, the TSR testing mill was developed for Through untiring efforts made over many years, the TSR testing mill was developed for experimental studies of seamless tubes, as shown in Figure2. This mill consists of the driving experimental studies of seamless tubes, as shown in Figure 2. This mill consists of the driving part of the piercing section, the main mill (the piercing section combined with the rolling section), the

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Metals 2020, 10, x FOR PEER REVIEW 3 of 11 part of the piercing section, the main mill (the piercing section combined with the rolling section), mandrelthe mandrel carriage carriage in which in which the the plug plug is joined is joined with with the the mandrel mandrel by by a ascrew screw thread, thread, and and the driving part of the rolling section.

Figure 2. TSR testing mill.

In order toto extendextend thethe rangerange ofof the the TSR TSR process, process, in in the the experimental experimental study, study, the the cylindrical cylindrical billets billets of carbon steel 1045, HSS 42CrMo, and magnesium alloy AZ31 were used with dimensions ϕ40 500 mm. of carbon steel 1045, HSS 42CrMo, and magnesium alloy AZ31 were used with dimensions ×φ40 × 500 Themm. rollersThe rollers in the in piercing the piercing section section were were the same the same sizes sizes with with a maximum a maximum diameter diameter of 180 of mm180 mm and anda length a length of 140 of 140 mm, mm, while while the the rollers rollers in in the the rolling rolling section section were were the the same same sizes sizeswith with aa diameterdiameter of 180 mm atat thethe shouldershoulder andand aa lengthlength ofof 120 120 mm. mm. TheThe plug plug was was used used together together with with the the mandrel mandrel with with a adiameter diameter of of 30 30 mm. mm. The The rollers rollers in in the the piercing piercing section,section, drivendriven byby A.C.A.C. motors,motors, rotated in the same direction withwith thethe samesame invariableinvariable rotary rotary speed speed of of 169 169 rpm. rpm. The The rollers rollers in in the the rolling rolling section, section, driven driven by byD.C. D.C. motors, motors, rotated rotated in the in samethe sa directionme direction with with the same the same variable vari rotaryable rotary speed, speed, which which should should match matchthe speed the ofspeed the rollers of the in rollers the piercing in the sectionpiercing under section diﬀ erentunder process different parameters. process parameters. The reasonable The reasonableranges of process ranges of parameters process parameters were obtained were throughobtained previousthrough previous multiple multiple experiments. experiments. The rolling The rollingtemperature temperature was very was important very important for hot plastic for hot deformation, plastic deformation, especially especially the alloy material.the alloy Therefore, material. Therefore,the initial rolling the initial temperatures rolling temperatures for steel 1045, for HSS steel 42CrMo, 1045, HSS and 42CrMo, AZ31 alloy and were AZ31 set allo at 1200,y were 1250, set and at 1200,400 ◦C, 1250, respectively, and 400 which °C, respectively, were selected towhich make were full use selected of their to plastic make deformation full use of capacity their plastic under deformationcorresponding capacity high temperatures. under corresponding The process high parameters temperatures. used in experimentsThe process arepara shownmeters in used Table in1. experiments are shown in Table 1. Table 1. Process parameters of the TSR testing mill.

ItemTable Piercing 1. SectionProcess parameters of the TSR testing mill. Rolling Section Feed Entrance Roll Plug Feed Entrance Roll Roll Process Item Angle FacePiercing Angle SectionGap Advance Angle Face Rolling Angle SectionGap Rotational Parameter (deg) Entrance(deg) (mm) (mm) (deg) Entrance(deg) (mm) Speed/rpm Feed Roll Plug Feed Roll Roll Adjustable Process 0–15Face 0–9 30–48 0–35 0–15Face 0–9 35–48 0–200 rang Angle Gap Advance Angle Gap Rotational Parameter Angle Angle Reasonable (deg) (mm) (mm) (deg) (mm) speed/rpm 7–9 0 34–36 20–25 7–9 4 37–39 174–190 rang (deg) (deg) AdjustableExperimental 0–158 0–9 0 30–48 35 0–35 23 0–15 9 0–9 4 35–48 39 0–200 186 valuesrang Reasonable 7–9 0 34–36 20–25 7–9 4 37–39 174–190 2.2. Finiterang Element Model Experimental For the analysis8 of the metal0 flow and35 deformation23 in the9 TSR process,4 the commercial39 software186 Deform-3Dvalues was applied. Due to the complexity, the analysis of the metal flow and deformation should be simulated only in a 3D state of strain conditions. In Figure3, the worked-out FEM of the TSR process 2.2. Finite Element Model according to the schema in Figure1 is presented. This model consists of three barrel-shaped rollers, threeFor tapered the analysis rollers, of a pusher,the metal a plugflow togetherand deformatio with a mandrel,n in the TSR and process, a billet. the The commercial billet is defined software as a plasticDeform-3D body, was and applied. the others Due are to modeled the complexity, as rigid bodies. the analysis Additionally, of the metal the pusher flow wasand useddeformation to place shouldthe billet be into simulated the area only between in a 3D rollers. state Theof strain process conditions. parameters In Figure (e.g., feed 3, the angle, worked-out entrance FEM face angle,of the TSRroll gap,process and speed)according used to in thethe FEMschema were in the Figure same as1 thoseis presented. used in the This experiment model consists and are shownof three in barrel-shaped rollers, three tapered rollers, a pusher, a plug together with a mandrel, and a billet. The billet is defined as a plastic body, and the others are modeled as rigid bodies. Additionally, the pusher was used to place the billet into the area between rollers. The process parameters (e.g., feed angle, entrance face angle, roll gap, and speed) used in the FEM were the same as those used in the

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Metals 2020, 10, x FOR PEER REVIEW 4 of 11 experiment and are shown in Table 1. However, a short distance of 200 mm between the piercing Metals 2020, 10, 59 4 of 11 sectionexperiment and the and rolling are shown section in was Table used 1. However,in the simulation a short indistance order to of save 200 computationmm between time, the piercing which wassection 630 mmand inthe the rolling TSR testingsection mill.was used in the simulation in order to save computation time, which Tablewas1 .630 However, mm in the a short TSR distancetesting mill. of 200 mm between the piercing section and the rolling section was used in the simulation in order to save computation time, which was 630 mm in the TSR testing mill.

Figure 3. Finite element model (FEM) of the TSR process. Figure 3. Finite element model (FEM) of the TSR process. Figure 3. Finite element model (FEM) of the TSR process. Aiming to reflect the metal flow and deformation rule, only the 1045 steel was selected in Aiming to reflect the metal flow and deformation rule, only the 1045 steel was selected in simulation. A rigid plastic material model, which is the 1045 steel of the software’s own model, was simulation.Aiming A rigidto reflect plastic the material metal model,flow and which deformatio is the 1045n rule, steel only of the the software’s 1045 steel own was model, selected was in employed and assigned to the billet with dimensions φ40 × 200 mm, whose flow curves are shown in employedsimulation. and A assigned rigid plastic to the material billet with model, dimensions which isϕ the40 1045200 steel mm, of whose the software’s flow curves own are model, shown was in Figure 4. The billet was meshed and approximately divided× into 30,000 tetrahedral elements. It was Figureemployed4. The and billet assigned was meshed to the andbillet approximately with dimensions divided φ40 × into 200 30,000mm, whose tetrahedral flow curves elements. are shown It was in assumed that the billet was heated to 1200 °C, and the temperatures of the rigid tools, the rollers, and assumedFigure 4. that The the billet billet was was meshed heated and to 1200 approximatelyC, and the temperaturesdivided into 30,000 of the rigidtetrahedral tools, theelements. rollers, It and was the pusher were constant at 150 °C during◦ the whole forming process. In the case of the plug and the theassumed pusher that were the constant billet was at 150 heatedC during to 1200 the °C, whole and th forminge temperatures process. of In the the rigid case tools, of the the plug rollers, and the and mandrel, it was assumed that th◦e temperature was the same as the environment temperature—20 °C. mandrel,the pusher it was were assumed constant that at 150 the temperature°C during the was whol thee sameforming as theprocess. environment In the case temperature—20 of the plug and C.the It has been stated that the coefficient of the heat exchange between the tools and formed material◦ is Itmandrel, has been it stated was assumed that the coethatffi thciente temperature of the heat was exchange the same between as the theenvironment tools and formedtemperature—20 material is °C. 11 N/(s·mm·C). However, the value of this coefficient determining the heat exchange between the 11It N has/(s mmbeenC). stated However, that the the coefficient value of this of coetheffi heatcient exchange determining between the heat the exchangetools and betweenformed material the billet is billet and· ·the environment was 0.02 N/(s·mm·C). Friction is an important factor for a smooth and11 theN/(s·mm·C). environment However, was 0.02 the N /(svalumme ofC). this Friction coefficient is an important determining factor the for heat a smooth exchange rolling between process. the rolling process. The greater the friction,· · the easier it is to roll smoothly, and the less time spent Thebillet greater and thethe friction,environment the easier was it 0.02 is to N/(s·mm·C). roll smoothly, Friction and the is less an time important spent in factor simulation. for a Duesmooth to theinrolling simulation. purposeful process. roughnessDue The to greaterthe ofpurposeful the the rollers, friction, roughness it was the assumed easier of the it rollers, that is to the roll it friction was smoothly, assumed factor and forthat thesethe the less friction rollers time hadfactor spent a limitingforin thesesimulation. value rollers of Duehad 1.0. ato However, limiting the purposeful value because of roughness 1.0. the Howe plug wasofver, the madebecause rollers, with theit was a plug follow-up assumed was made rotational that with the friction a movement follow-up factor withrotationalfor thethese billet movementrollers during had piercing, witha limiting the thebillet value friction during of of1.0. piercing the Howe surfacever,, the of becausefriction contact ofthe between the plug surface was theplug ofmade contact and with the between a billet follow-up was the veryplugrotational low. and Therefore,the movement billet was the with very friction the low. billet was Therefore, set during to 0.3 piercingthe in thefriction FEM., the was friction set to of 0.3 the in surface the FEM. of contact between the plug and the billet was very low. Therefore, the friction was set to 0.3 in the FEM. 240 400 （a） 0.1s-1 （b） 100s-1 900℃ 900℃ 200240 350400 （a） 0.1s-1 （b） 100s-1 900℃ 300 900℃ 350 ℃ 150200 1000℃ 1000 250300 ℃ 150 1000℃ 11001000℃ 100 1100℃ 200250 150 1100℃ 100 12001100℃℃ 200 1200℃ 50 100 1200℃ 150 1200℃ 50 0 50100 0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5 0 ε 50 ε 0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5 (a) ε 1(b) ε 1 Figure 4. Flow curves of 1045 steel at diff erent strain rates: (a) 0.1 s− and (b) 100 s− . 3. Results andFigure Discussion 4. Flow(a )curves of 1045 steel at different strain rates: (a) 0.1 s−1 (andb) (b) 100 s−1. Figure 4. Flow curves of 1045 steel at different strain rates: (a) 0.1 s−1 and (b) 100 s−1. 3.1.3. Results Numerical and Modeling Discussion Results of the Steel 1045 during the TSR Process 3. Results and Discussion 3.1. NumericalThe changes Modeling in the billetResults shape of th duringe Steel 1045 the TSRduring process the TSR are Process presented in Figure5, from which two rollers are omitted to improve readability. 3.1. TheNumerical changes Modeling in the billetResults shape of the during Steel 1045 the during TSR process the TSR are Process presented in Figure 5, from which two rollersThe changes are omitted in the to billetimprove shape readability. during the TSR process are presented in Figure 5, from which two rollers are omitted to improve readability.

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Figure 5. ChangeChange in billet shape du duringring the TSR process. Figure 5. Change in billet shape during the TSR process. Based on Figure5 5,, atat the the beginning,beginning, thethe billetbillet isis clampedclamped byby rollersrollers inin thethe piercingpiercing section,section, whichBased puts iton into Figure a rotary 5, at motion the beginning, and later transmitstransmitthe billets itis inclamped the axial by direction. rollers in A theprocess piercing of rotational section, compressionwhich puts it is into then a rotarystarted, motion and this and lasts later until transmit the the head heads it inof of thethe the axialbillet billet direction. has has made made A contact contact process with with of rotational the the plug. plug. Next,compression the forming is then process started, of and the this hollow lasts shell until internal the head hole of thebegins, billet and has its made size contact is determined with the by plug. the appliedNext, the plug forming dimension. process At Atof approximately approximatelythe hollow shell Step Step internal 490 4900,0, holethe the head headbegins, of andthe hollow its size isshell determined moves straight by the outapplied from plug the rollers dimension. in the Atpiercing approximately section. Further, Further, Step 490 with 0, the continuous head of therolling, hollow the shellpierced moves hollow straight shell makesout from contact the rollers with the in the rollers piercing in the section. rolling Further, section at with approximately continuous rolling, Step 5600 the and pierced is rolled hollow into shell the rollmakes gap. contact Under Under with the the combination combinationthe rollers in ofthe of the the rolling roller roller section in the atpiercing approximately section and Step the 5600 rolling and issection, rolled the into TSR the processroll gap. reaches Under athe stable combination phase after of Stepthe roller 6000.6000. in the piercing section and the rolling section, the TSR processThe reaches distribution a stable of phasestress after determined Step 6000. for the stable stage in the longitudinal and cross-section planesThe isis presenteddistributionpresented inin Figureof Figure stress6, 6, anddetermined and the the strain strain for is presentedtheis presented stable instage Figurein Figurein 7the. The longitudinal7. The data data in Figure in and Figure6 cross-sectionshows 6 shows that thathigherplanes higher stressis presented stress intensity intensity in magnitudesFigure magnitudes 6, and occur the atstrainoccur locations isat presentedlocations outside whereoutsidein Figure the where surface7. The the data contacts surface in Figure the contacts rollers 6 shows andthe rollersarethat equal higher and to approximatelystressare equal intensity to approximately 160magnitudes MPa in both occur160 sections, MPaat locations in and both the outside sections, highest wherevalues and thethe are surfacehighest concentrated contacts values at are the concentratedshoulderrollers and of theare at rollerstheequal shoulder into theapproximately rollingof the section.rollers 160 in Meanwhile, MPathe rollingin both the section. changessections, Meanwhile, inand the the diﬀ erentthehighest changes cross values sections in theare differentindicateconcentrated that cross it at makessections the shoulder the indicate sharpest of that contactthe rollersit makes only in at ththe thee sharpest momentrolling section. whencontact the Meanwhile,only rollers at bitethe themoment billetchanges fromwhen in three the rollersdirections.different bite cross Asthe thesectionsbillet billet from indicate moves three along directions.that it the makes deformation As ththee sharpestbillet region, moves contact the along magnitude only the at deformation the of themoment stress region, when intensity, the whilemagnituderollers it bite deepens ofthe the billet fromstress from outside intensity, three to inside,directions.while it increases deepens As the withfrom billet anoutside moves increase to along inside, in the the contactincreases deformation surface. with anregion, The increase same the magnitude of the stress intensity, while it deepens from outside to inside, increases with an increase inchange the contact regularity surface. is presented The same in change the rolling regularity section. is presented in the rolling section. in the contact surface. The same change regularity is presented in the rolling section.

Figure 6. StressStress distribution distribution in the stable st stageage (Step 6000) of the TSR process. Figure 6. Stress distribution in the stable stage (Step 6000) of the TSR process.

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Figure 7. Strain distribution in the stable stage (Step 6000) of the TSR process. Figure 7. Strain distribution in the stable stage (Step 6000) of the TSR process. In Figure 7,Figure it can 7. Strainbe observed distribution that in strainthe stables are stage presented (Step 6000) in ofa thelaminar TSR process. way. However, the greatestIn Figurestrains7 ,are it canpresent be observed in the external that strains layers are of presented the formed in atube. laminar In Figure way. However, 7, the changes the greatest in the In Figure 7, it can be observed that strains are presented in a laminar way. However, the billetstrains cross are profile present along in the the external whole layers length of of the the formed forming tube. area In can Figure be 7analyzed., the changes Due in to the the billet influence cross greatest strains are present in the external layers of the formed tube. In Figure 7, the changes in the ofprofile the rollers, along thethe wholecross profile length varies of the formingfrom a circle area canto a betriangular analyzed. circle Due and to the then influence to another of the circle rollers, in billet cross profile along the whole length of the forming area can be analyzed. Due to the influence thethe piercing cross profile section. varies The fromcross aprofile circle continuously to a triangular varies circle from and a then circle to to another a triangular circle circle in the and piercing then of the rollers, the cross profile varies from a circle to a triangular circle and then to another circle in tosection. another The circle cross in profilethe rolling continuously section. Finally, varies fromafter the a circle metal to moves a triangular through circle the and mandrel, then to the another tube the piercing section. The cross profile continuously varies from a circle to a triangular circle and then gainscircle the in therequired rolling circular section. shape. Finally, after the metal moves through the mandrel, the tube gains the to another circle in the rolling section. Finally, after the metal moves through the mandrel, the tube requiredTo clearly circular display shape. the distributions of stress and strain determined for the stable stage, Figure 8 gains the required circular shape. showsTo an clearly isometric display view the and distributions longitudinal of section stress andof distribution strain determined patternsfor for thethe stableseamless stage, steel Figure tubes8 To clearly display the distributions of stress and strain determined for the stable stage, Figure 8 duringshows the an isometricTSR process view as and follows: longitudinal (a) stress, section (b) effective of distribution strain, (c) patterns velocity, for and the seamless(d) velocity steel with tubes a shows an isometric view and longitudinal section of distribution patterns for the seamless steel tubes vectorduring plot, the in TSR which process the asdistributions follows: (a) of stress, the velocity (b) effective field and strain, metal (c) flow velocity, are obtained. and (d) velocity with a vectorduring plot,the TSR in which process the as distributions follows: (a) ofstress, the velocity (b) effective field andstrain, metal (c) velocity, flow are obtained.and (d) velocity with a vector plot, in which the distributions of the velocity field and metal flow are obtained.

(a) (b) (a) (b)

FigureFigure 8. 8. DistributionDistribution (patternc) pattern of ofthe the producing producing seam seamlessless steel steel tube tube in the in theTSR TSR (process:d) process: (a) stress, (a) stress, (b) effective(b) eﬀective strain, strain, (c) velocity, (c) velocity, and and(d) velocity (d) velocity with with a vector a vector plot. plot. Figure 8. Distribution pattern of the producing seamless steel tube in the TSR process: (a) stress, (b) effective strain, (c) velocity, and (d) velocity with a vector plot.

Metals 2020, 10, x FOR PEER REVIEW 7 of 11 Metals 2020, 10, 59 7 of 11 In Figure 8c, distribution law is arranged in layers of the velocity fields, which show a decreasing trend from outside to inside. The highest magnitude appears at the outside surface, while the lowestIn Figure magnitude8c, distribution appears law atis the arranged central inaxial layers zone of thein front velocity of the fields, plug. which When show the a head decreasing of the trendbillet touches from outside the plug, to inside. the core The metal highest flows magnitude outward appearsalong the at surface the outside of the surface, plug under while the the friction lowest magnitudeforce given appearsby the rollers. at the centralThis force axial was zone enough in front to present of the plug. the metal When flow the headrule of of the the tube billet obtained touches theby TSR plug, process. the core Figure metal flows8d gives outward the velocity along thevector surface of the of tube the plug produced under in the the friction process force and given shows by the rollers.metal flow This rule. force The was velocity enough vector to present is always the metal tangential flow rule to the of thesurface tube and obtained forms by an TSR angle process. with Figurethe axial8d givesline, which the velocity determines vector the of the movement tube produced pattern in of the the process billet. and When shows touching the metal the flowplug, rule. the Thecore velocitymetal flows vector outward is always along tangential the surface to the of surface the plug. and Note forms that an angleno cavity with is the formed axial line,in the which core determinesmetal in front the of movement the plug. pattern of the billet. When touching the plug, the core metal flows outward along the surface of the plug. Note that no cavity is formed in the core metal in front of the plug. 3.2. Experimental Analysis 3.2. Experimental Analysis 3.2.1. Quality of Tubes 3.2.1. Quality of Tubes Figure 9 shows the seamless tubes produced on the TSR testing mill. As shown in Figure 9, the Figure9 shows the seamless tubes produced on the TSR testing mill. As shown in Figure9, tubes of carbon steel 1045 and AZ31 alloy achieved the desired cylindrical shapes. However, HSS the tubes of carbon steel 1045 and AZ31 alloy achieved the desired cylindrical shapes. However, HSS 42CrMo could not be successfully completed with the rear-jamming of the tail of the billet present in 42CrMo could not be successfully completed with the rear-jamming of the tail of the billet present in the the piercing section. The season was due to the collapsing nose of the plug, which resulted in the piercing section. The season was due to the collapsing nose of the plug, which resulted in the increase increase in axial resistance. Meanwhile, there were no significant defects on the outside surfaces of in axial resistance. Meanwhile, there were no significant defects on the outside surfaces of the tubes, the tubes, except from the tail burrs of the AZ31 alloy and the trumpet-shaped heads of the tubes. In except from the tail burrs of the AZ31 alloy and the trumpet-shaped heads of the tubes. In addition, addition, the carbon steel 1045 tubes had diameters of 39.14 ± 0.35 mm, wall thicknesses of 4.67 ± 0.2 the carbon steel 1045 tubes had diameters of 39.14 0.35 mm, wall thicknesses of 4.67 0.2 mm, mm, and lengths of up to approximately 1360 mm, and± the dimensions of the AZ31 alloy tubes± were and lengths of up to approximately 1360 mm, and the dimensions of the AZ31 alloy tubes were 39.85 ± 0.35 mm, 4.42 ± 0.25 mm, and 1200 mm, respectively. However, only a length of 750 mm of 39.85 0.35 mm, 4.42 0.25 mm, and 1200 mm, respectively. However, only a length of 750 mm of HSS 42CrMo± tube was± obtained and rolled by the TSR process, and the diameters and wall HSS 42CrMo tube was obtained and rolled by the TSR process, and the diameters and wall thicknesses thicknesses were about 38.22 ± 0.35 mm and 4.33 ± 0.3 mm, respectively. The comparative data of were about 38.22 0.35 mm and 4.33 0.3 mm, respectively. The comparative data of these tubes are these tubes are listed± in Table 2. ± listed in Table2.

Figure 9. Seamless tubes produced in the TSR testing mill. Figure 9. Seamless tubes produced in the TSR testing mill. Table 2. The dimensional values of the tubes with different materials by experiments. Table 2. The dimensional values of the tubes with different materials by experiments. Material Diameter (mm) Thickness (mm) Length (mm) Extension Coefficient Carbon steel 1045 39.14 0.35 4.67 0.20 1360Extension 2.72 Material Diameter± (mm) Thickness± (mm) Length (mm) HSS 42CrMo 38.22 0.4 4.33 0.30 1400Coefficient 2.80 ± ± AZ31Carbon alloy steel 1045 39.85 39.140.35 ± 0.35 4.42 4.670.25 ± 0.20 1200 1360 2.72 2.40 ± ± HSS 42CrMo 38.22 ± 0.4 4.33 ± 0.30 1400 2.80 The distributionAZ31 alloy of thicknesses 39.85 ± 0.35 of the tubes 4.42 along ± 0.25 the axial direction 1200 is shown 2.40 in Figure 10. Figure 10 shows that the tubes of carbon steel 1045 and the AZ31 alloy had a more accurate thickness comparedThe distribution with the HSS of 42CrMothicknesses tube of produced the tubes in al theong TSR the testing axial mill.direction Meanwhile, is shown under in Figure the same 10. processFigure 10 parameters shows that in the the tubes experiment, of carbon diff steelerent 104 diameters5 and the and AZ31 thicknesses alloy had were a more obtained accurate with thickness different materialscompared andwith properties. the HSS 42CrMo The extension tube produced coefficient in the of TSR the carbontesting mill. steel Meanwhile, 1045 was about under 2.72, the whilesame thoseprocess of HSSparameters 42CrMo in and the the experiment, AZ31 alloy different were about diameters 2.8 and and 2.4, respectively.thicknesses were The reasonsobtained for with this mightdifferent be materials linked directly and properties. to the plastic The extension deformation coefficient properties of the themselves. carbon steel From 1045 the was condition about 2.72, of experiments,while those of the HSS color 42CrMo of the and tandem the AZ31 rolled alloy tube were of the about HSS 42CrMo2.8 and 2.4, darkened respectively. more quickly The reasons than that for ofthis the might carbon be steellinked 1045, directly which to was the suplasticfficient deform to proveation that properties HSS 42CrMo themselves was a temperature-responsive. From the condition of

Metals 2020, 10, x FOR PEER REVIEW 8 of 11

Metals 2020, 10, 59 8 of 11 experiments, the color of the tandem rolled tube of the HSS 42CrMo darkened more quickly than that of the carbon steel 1045, which was sufficient to prove that HSS 42CrMo was a temperature-responsivemetal. Under a proper rolling metal. temperature,Under a proper the roll AZ31ing alloytemperature, tube was the rolled AZ31 more alloy easily tube thanwas others.rolled moreAdditionally, easily than a uniform others. wall Addition thicknessally, ofa uniform the middle wall section thickness of the of tubes the middle was obtained, section the of reasonthe tubes for waswhich obtained, might be the the reason stable for rolling which in might the process. be the stable rolling in the process.

FigureFigure 10. 10. DistributionDistribution of of the the thickness thickness in in the the axial axial direction. direction. 3.2.2. Roll Force 3.2.2. Roll Force Figure 11 shows the experimental data related to the roll force of the two sections and the plug Figure 11 shows the experimental data related to the roll force of the two sections and the plug axial force in the TSR process for the production of seamless tubes: (a) carbon steel 1045, (b) magnesium axial force in the TSR process for the production of seamless tubes: (a) carbon steel 1045, (b) alloy AZ31, and (c) HSS 42CrMo, respectively. Because of the plug combined with the mandrel, the date magnesium alloy AZ31, and (c) HSS 42CrMo, respectively. Because of the plug combined with the of the axial force could be the sum of the dates on both. As shown in Figure 11a, the whole process of mandrel, the date of the axial force could be the sum of the dates on both. As shown in Figure 11a, the rolling carbon steel 1045 can be easily divided into three stages: (i) piercing, (ii) piercing and rolling, the whole process of the rolling carbon steel 1045 can be easily divided into three stages: (i) piercing, and (iii) rolling, which is the same as the AZ31 alloy. However, there was no stage of rolling solely in (ii) piercing and rolling, and (iii) rolling, which is the same as the AZ31 alloy. However, there was no Figure 11c because of the rear-jamming present during the rolling tube of HSS 42CrMo. What could be stage of rolling solely in Figure 11c because of the rear-jamming present during the rolling tube of seen in Figure 11 is that the roll force values of both sections of the rolling HSS 42CrMo are greater HSS 42CrMo. What could be seen in Figure 11 is that the roll force values of both sections of the than those of the rolling carbon steel 1045 and the AZ31 alloy, respectively. The roll force of the AZ31 rolling HSS 42CrMo are greater than those of the rolling carbon steel 1045 and the AZ31 alloy, alloy is the smallest. As is known, compared with carbon steel, HSS has a higher tensile strength when respectively. The roll force of the AZ31 alloy is the smallest. As is known, compared with carbon small amounts of alloying elements are added, while magnesium alloy has a lower one. steel, HSS has a higher tensile strength when small amounts of alloying elements are added, while As shown in Figure 11a,b, with the billet pushed and contacted with the rollers of the piercing magnesium alloy has a lower one. section, the roll force of the piercing section increased steeply in stage (i). In quick succession, when the As shown in Figure 11a and Figure 11b, with the billet pushed and contacted with the rollers of head of the incoming billet made contact with and passed through the plug, the roll force of the piercing the piercing section, the roll force of the piercing section increased steeply in stage (i). In quick section slightly rose and then reached a relative steady state, followed by a slight decline. Meanwhile, succession, when the head of the incoming billet made contact with and passed through the plug, the axial force on the plug also increased and then remained stable. the roll force of the piercing section slightly rose and then reached a relative steady state, followed Continuously, when the pierced hollow tube moved forward in a spiral fashion and fed into the by a slight decline. Meanwhile, the axial force on the plug also increased and then remained stable. roll gap of the rolling section, it was situated in the real TSR process in stage (ii). The roll force of the rolling section increased rapidly and afterwards decreased slowly to a certain extent. Meanwhile, the force of the two sections featured a downward sloping curve on a small scale, because of the interstand tension between the two sections. The axial force also increased with the addition of force on the mandrel. In stage (iii), as the billet exited the roll gap of the piercing section, the force of the piercing section decreased steeply, and the axial force decreased rapidly to a small value because of the lack of axial force on the plug. Meanwhile, the force of the rolling section slightly increased due to the lack of the interstand tension mentioned above. The force decreased to 0 when completed. As shown in Figure 11c, the rolling HSS 42CrMo took longer than the carbon steel 1045, which reﬂects the diﬃculty of rolling high-deforming steel, such as 42CrMo. Based on Figure 11c, the slip between the roller and the billet in the piercing section occurred at about 12 s, accompanied by the increase in the force of the piercing section, and no axial movement occurred thereafter in the actual experiment. The force decreased to 0 after a manual stop. The reason for this is mentioned (a) (b) above: the collapsing nose of the plug and the increase of axial resistance must be a major factor causing the rear-jamming, which increased the force of axial resistance, even with greater friction. As is known, friction between the billet and the roller is key in the rolling process. Meanwhile, the material characteristics of HSS 42CrMo in the deformation may be an intrinsic factor, which is determined

Metals 2020, 10, x FOR PEER REVIEW 8 of 11 experiments, the color of the tandem rolled tube of the HSS 42CrMo darkened more quickly than that of the carbon steel 1045, which was sufficient to prove that HSS 42CrMo was a temperature-responsive metal. Under a proper rolling temperature, the AZ31 alloy tube was rolled more easily than others. Additionally, a uniform wall thickness of the middle section of the tubes was obtained, the reason for which might be the stable rolling in the process.

Metals 2020, 10, 59 9 of 11 Figure 10. Distribution of the thickness in the axial direction. by3.2.2. the Roll small Force amounts of alloying elements. Even so, this is enough to prove the feasibility of the TSR process for HSS seamless tubes. The rolling process thus might become smoother in two ways: the rollingFigure velocity 11 shows might the increase,experimental and thedata roll related friction to the might roll increase.force of the These two methods, sections and under the these plug conditions,axial force couldin the therefore TSR process decrease for the the work production time of plugs.of seamless tubes: (a) carbon steel 1045, (b) magnesiumAs shown alloy in FigureAZ31, 11andb, (c) the HSS variation 42CrMo, tendency respectively. of the rollBecause force of the plug AZ31 combined alloy was with similar the tomandrel, the one the of steel date 1045. of the Only axial the force used could time be of the each su deformationm of the dates section on both. was As shorter shown than in Figure steel 1045, 11a, andthe whole the average process force of the values rolling were carbon lower. steel Based 1045 on ca othern be easily research divided on piercing into three experiments stages: (i) withpiercing, the AZ31(ii) piercing alloy [20 and,21 rolling,], using and the TSR(iii) rolling, process which to manufacture is the same seamless as the AZ31 tubes alloy. of AZ31 However, alloys was there a daringwas no attemptstage of and rolling has beensolely proven in Figure to be 11c a breakthrough.because of the rear-jamming present during the rolling tube of HSSGenerally, 42CrMo. What this could could bebe usedseen asin aFigure reference 11 is for that similar the roll kinds force of values material. of both Conversely, sections of there the mightrolling be HSS some 42CrMo distinctions are greater for diff thanerent those kinds of of the materials, rolling incarbon which steel some 1045 factors and a fftheect AZ31 the results, alloy, e.g.,respectively. deformation The temperature, roll force of deformationthe AZ31 alloy rate, is and the deformationsmallest. As degree.is known, compared with carbon steel,Fortunately, HSS has a higher different tensile material strength productions when small for tubes amounts formed of alloying by TSR haveelements been are successful, added, while and themagnesium process has alloy been has shown a lower to one. be feasible for use. Further tests where the plug is replaced should be done,As with shown the goal in Figure of achieving 11a and a Figure longer 11b, contact with time the billet under pushed high temperatures. and contacted Adjusting with the rollers process of parametersthe piercing is section, another the effective roll force way ofof increasingthe piercing the section rolling increased velocity or steeply shortening in stage the contact(i). In quick time betweensuccession, the when plug andthe head the billet. of the The incoming emphasis billet should made be contact on high-quality with and passed tubes of through nonferrous the plug, and high-deformingthe roll force of metals,the piercing suchas section tubes ofslightly titanium rose alloy and usedthen inreached oil well a fields,relative and steady such highstate,quality followed is theby ultimatea slight decline. goal of Meanwhile, the TSR process. the axial force on the plug also increased and then remained stable.

Metals 2020, 10, x FOR PEER REVIEW 9 of 11 (a) (b) 80 75 42CrMo 1 Piercing Section Roll Force 70 2 Rolling Section Roll Force 65 3 Axial Force 60 1 55 50 Piercing 45 Piercing +Rolling 40 35 30 25 20 2 15 3 10 5 0 0 123456789101112 13 14 15 16 17 18 19 20 Time /s (c)

Figure 11. ExperimentalExperimental data data relating relating roll roll force force an andd plug plug axial axial force force in inthe the TSR TSR process: process: (a) 1045, (a) 1045, (b) (AZ31,b) AZ31, and and (c) 42CrMo. (c) 42CrMo.

Continuously, when the pierced hollow tube moved forward in a spiral fashion and fed into the roll gap of the rolling section, it was situated in the real TSR process in stage (ii). The roll force of the rolling section increased rapidly and afterwards decreased slowly to a certain extent. Meanwhile, the force of the two sections featured a downward sloping curve on a small scale, because of the interstand tension between the two sections. The axial force also increased with the addition of force on the mandrel. In stage (iii), as the billet exited the roll gap of the piercing section, the force of the piercing section decreased steeply, and the axial force decreased rapidly to a small value because of the lack of axial force on the plug. Meanwhile, the force of the rolling section slightly increased due to the lack of the interstand tension mentioned above. The force decreased to 0 when completed. As shown in Figure 11c, the rolling HSS 42CrMo took longer than the carbon steel 1045, which reflects the difficulty of rolling high-deforming steel, such as 42CrMo. Based on Figure 11c, the slip between the roller and the billet in the piercing section occurred at about 12 s, accompanied by the increase in the force of the piercing section, and no axial movement occurred thereafter in the actual experiment. The force decreased to 0 after a manual stop. The reason for this is mentioned above: the collapsing nose of the plug and the increase of axial resistance must be a major factor causing the rear-jamming, which increased the force of axial resistance, even with greater friction. As is known, friction between the billet and the roller is key in the rolling process. Meanwhile, the material characteristics of HSS 42CrMo in the deformation may be an intrinsic factor, which is determined by the small amounts of alloying elements. Even so, this is enough to prove the feasibility of the TSR process for HSS seamless tubes. The rolling process thus might become smoother in two ways: the rolling velocity might increase, and the roll friction might increase. These methods, under these conditions, could therefore decrease the work time of plugs. As shown in Figure 11b, the variation tendency of the roll force of the AZ31 alloy was similar to the one of steel 1045. Only the used time of each deformation section was shorter than steel 1045, and the average force values were lower. Based on other research on piercing experiments with the AZ31 alloy [20,21], using the TSR process to manufacture seamless tubes of AZ31 alloys was a daring attempt and has been proven to be a breakthrough. Generally, this could be used as a reference for similar kinds of material. Conversely, there might be some distinctions for different kinds of materials, in which some factors affect the results, e.g., deformation temperature, deformation rate, and deformation degree. Fortunately, different material productions for tubes formed by TSR have been successful, and the process has been shown to be feasible for use. Further tests where the plug is replaced should be done, with the goal of achieving a longer contact time under high temperatures. Adjusting process parameters is another effective way of increasing the rolling velocity or shortening the contact time between the plug and the billet. The emphasis should be on high-quality tubes of nonferrous and

Metals 2020, 10, 59 10 of 11

4. Conclusions Based on the studies carried out, it was found that rolling on a TSR testing mill allowed TSR to be run with high eﬃciency in the deformation of seamless tubes pierced and rolled at the same time time, which makes continuous skew rolling more economical and practical. The following conclusions can be drawn.

(1) The distribution of stress presented higher stress intensity magnitudes on outside surfaces contacting with the rollers. (2) The distribution of strain was presented in a laminar way, with higher strain intensity magnitudes occurring at the external layers of the formed tube. The cross proﬁle varied from a circle to a triangular circle and then to another circle in the piercing section, and again, continuously, in the rolling section. (3) The distribution of velocity was also arranged in layers with a higher magnitude at the outside surface and the lowest one at the central axial zone in front of the plug. (4) Based on experiments in a TSR testing mill, the desired seamless tubes of carbon steel 1045, HSS 42CrMo, and the AZ31 alloy were obtained, with an accuracy of 0.2 mm in wall thickness ± and a diameter of 0.35 mm. ± (5) The TSR process is suitable for manufacturing seamless tubes that are diﬃcult to deform or that are deformed in a narrow temperature range.

Author Contributions: Y.S. and F.W. conceived and designed the experiments; F.M., J.H., and J.C. performed the experiments; F.M. and J.H. analyzed the data; F.M. performed the finite element simulations and wrote the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi grant number [2019L0852], the Major Project of the Ministry of Science and Technology of Shanxi Province in China grant number [20191102009], and the Start-Up Foundation for Doctors of Yuncheng University grant number [YQ–2019006]. And the APC was funded by [YQ–2019006]. Acknowledgments: The authors acknowledge the Funds for the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (No. 2019L0852), the Major Project of the Ministry of Science and Technology of Shanxi Province, China (No. 20191102009), and the Start-Up Foundation for Doctors of Yuncheng University (No.YQ–2019006). Conflicts of Interest: The authors declare that there is no conflict of interest.

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