Groove Design and Microstructure Research of UltraFine Grain Bar Rolling *

New flat-oval groove rolling process of multi-direction deformation is proposed to manufacture ultra-fine grain bar. Application of new groove series can introduce uniform large plastic strain into whole cross section of the material, and meanwhile satisfy the requirements of shape and size. Principle of grain refinement, based on experimental research of small specimen, is that grain refinement of ferrite is mainly dynamic recrystallization when low-carbon alloy steel is at low temperature deformation. Relationship of grain size and z-factor is also obtained through experimental research, as well as ultra-fine ferrite grain less than 1 micron. To predict strain, shape, dimensions and grain size of the material in rolling process, numerical simulation model of the warm groove bar rolling process is established via nonlinear finite element method, and distribution of grain size of the final section is obtained via finite element subroutine. The result indicates that ultra-fine grain bar rolling can accomplish at low temperature region.


Introduction
After entering the 21st century, countries in the world begin to concentrate on the study of fine-grain strengthening, such as "Super Steel" Project in Japan, "Highperformance Structural Steel in 21st Century" Project in Korea, and the national 973 project "Major Basic Research of New Generation Steel and Iron Materials" and et al. in China [1], which have made great breakthroughs in theory, and achieved a high degree of economic efficiency.Research shows that effective ways to manufacture ultra-fine grain is to reduce the deformation temperature and increase the plastic deformation.At present, it is an effective way to improve product performance obtaining steel of ultra-fine grain organization through control of thermal physical process, which avoids disadvantages of previous way of improving the properties of steel by adding alloying elements, such as rising costs, increased difficulty in smelting, poor property in welding.Steel of ultra-fine grained microstructures has excellent mechanical properties, such as high strength and good low-temperature impact toughness, which will be widely used as engineering structure materials for the future [2][3][4].There are several factors relating to refining of iron and steel material microstructures, among which large strain is necessary condition to produce ultra-fine grain microstructures.And there have been a number of reports of multi-pass and severe plastic deformation technologies in access to ultra-fine grain, such as equal channel angular pressing method, high pressure torsion method, and accumulative roll bonding method [5][6][7].
Bar caliber continuous rolling technology is very suitable for making ultra-fine grain microstructures, because of characteristic of large strain in multi-pass and multidirection manufacturing.The groove design is the key link in bar production, which directly relates to the rationality of metal deformation and products' microstructures and properties after rolling.In the present in order to improve the precision and efficiency of groove design, finite element method has been used more and more widely in metal plastic working field, and gradually become effective method in the rolling process of scientific prediction, process optimization and quantitative control Xu et al. [8] analysed the rolled piece shape size, temperature field, strain field and stress field in the bar rolling process on any position and anytime through finite element method, Mei et al. [9] by using finite element method analyzed the temperature distribution between hot strip and roller in different conditions, Wang et al. [10] combined finite element method with physical metallurgy which not only can predict the temperature, strain and stress distribution, but also can forecast the law of X. T. LI ET AL. 68 microstructure evolution in the process of metal forming that solved the problems in industrial production, Inoue et al. [11] used numerical simulation to design groove system, to calculate the strain and temperature distribution of bar rolling process, and then observed microstructure by practical rolling which agreed with simulation results.The trend of new groove and new technology is to get large strain of cross section by technology of improvement and innovation of groove design, and realize ultra-fine grain organization, especially to design new groove by numerical simulation and test and verify by experiments.
In recent years, a lot of research work on ultra-fine grain steel bar rolling technology has been carried out [12,13].In order to obtain ultra-fine grain steel products, added alloying elements and controlled method implementations are used, such as mild cooling-controlled process on grain refinement and strength [12].However, similarity of these researches is in high temperature austenitic region, while there has been rare report of obtaining smaller ultra-fine grains products by grain refinement in bar rolling process via warm processing method in ferrite region [14].
Hence this article presents to introduce internal quality factors into the groove design, and through experimental study on small specimens, the relationship between grain size and processing parameters is obtained.And numerical simulation technology is used to guide groove design to utmost meet deformation parameters that obtain ultrafine grain on condition that the precision criteria are met.Based on finite element analysis model of bar continuous rolling warm forming, deformation evolution rules of traditional groove and new flat-oval groove in rolling process are compared to conclude that the new type of flatoval groove can better introduce large strain into center of the section than the traditional groove, and its multidirectional plastic strain value meets conditions of ultrafine grain.

The Finite Element Model of Continuous Rolling
The coupled thermo-mechanical finite element model of multi-passes continuous rolling bar was established based on the platform of Abaqus as shown in Figure 1, where the mill rolls were rigid body and the rolled piece was elastic-plastic deformable body.They are meshed using 8-noded hexahedron elements.
Because the rolling temperature must be kept ferrite region at low temperature, the initial temperature of rolled piece was 450˚C.The temperature of mill roll was 200˚C and the temperature of air was 30˚C.The surface radiation coefficient of rolled piece was 0.8.Due to a lot of cooling water in the practical rolling process, the convection heat transfer coefficient was empirically given 40 kW/m 2 ˚C.The thermal contact conductance coefficient between rolled piece and mill roll was given 10 kW/m 2 ˚C.The contact friction model obeyed the coulomb's law of friction and the friction coefficient was given 0.3.The rolling velocity was 2500 mm/s.

Traditional Groove Discussion and Finite Element Simulation Analysis
Traditional groove design was based on the premise of hot rolling and shape controlling.Figure 2 is rhomb/ square groove system which is used to roll square steel of quadrate section, and the direction of arrow is the direction of rolling.The initial cross section shape of rolled piece of was quadrate of 24 mm by side, and 6 mm by curvature radius.In 450˚C and after 8 passes rolling, the cross section shape of final rolled piece was 13 mm by side.The accumulation of the strain distribution rule was gotten by the finite element simulation, and the accumulation of strain distribution of final cross section is shown in Figure 3(a).
In addition, this paper applies square/square groove shape to simulate the rolling process.Figure 3(b) is the rolling result of the finite element simulation of square/ square groove shape system, of which the initial conditions were the same as rhomb/square groove system.After 7 passes rolling the cross section shape of final piece was quadrate of 13 mm by side.In the square/square groove system "ears" appeared at the corners of the rolled piece, which were affected by the broadsiding conditions.In the subsequent rolling the phenomenon of fold easily appeared.
The above finite element simulation results show that due to influence by biting conditions and shape control, small strain appeared in the centre of rolled piece and large strain appeared on the edge of rolled piece in traditional groove rolling.As a result, there was restriction to get large plastic strain when making ultra-fine grain bar used traditional groove system.

The Design of the New Flat-Oval Groove
The flat-oval/square groove system proposed in this paper which has feature of so great elongation coefficient that can reduce the number of rolling passes.Meanwhile, in the rolling process the rhomb edge and side edge converted with each other which made the effect multi-direction processing more apparent than rhomb/square groove system.The rolled piece was first rolled by flat-oval groove and then by square groove which can achieve multi-direction and not simultaneously process and more effectively import large plastic strain into centre of materials.
In order to roll the square steel with the initial cross section shape of side of 24 mm, the curvature radius of 6 mm to square steel with final cross section shape of side of 18 mm through two passes of flat-oval/square groove system, this paper designed three kinds of flat-oval/ square groove system, the height of the flat-oval grooves were 8 mm, 12 mm, 16 mm respectively, the widths were the same, and the square grooves were all the same in shape.The rolling process of three grooves system was simulated using finite element method, and the accumulation of strain distribution is shown in Figure 4.In the graph the arrow direction is the rolling direction.
From the strain distribution in Figure 4, it can be clearly seen that when the flat-oval groove height (H) was 8 mm, the entire section introduced the largest deformation among three grooves system because of big rolling reduction.When the rolled piece went through the second pass of square groove, due to the influence of the pattern of metal plastic flow the groove cannot be totally filled and the strain cannot reach the centre of rolled piece.When the flat-oval groove height (H) was 12 mm, the cross section was smaller after the first rolling pass, but after the second square groove the largest stain in the centre of rolled piece and the groove can be totally filled.When the flat-oval groove height (H) was 16 mm, due to duced small strain.After the second rolling pass the small rolling reduction the whole cross section intro lement si

Experimental Study on Microstructure
s article is based on exis 0 -4, and in the same specimen, microstructure and strain cannot reach the centre of rolled piece and fullness degree of the rolled piece was large."Ears" appeared in the roll gap which was not beneficial to next rolling.Therefore, the flat-oval groove height (H) of 12 mm not only realized the cross section centre to get large strain but also guaranteed the rolled piece to totally filled of groove and realized to control the shape and size.
Based on the result of several times of finite e mulations, the author proposed new groove system of getting large strain as shown in Figure 5.The blank of square steel with side of 24 mm, flipped 90˚ after first of flat-oval groove, passed into the second pass of square groove with side of 18 mm, then flipped 45˚ passed into the third pass of flat-oval groove, then flipped 90˚ passed into the fourth pass of square groove with side of 13 mm, and then flipped 45˚ passed into the fifth pass of precision rolling of flat-oval groove, and finally flipped 90˚ passed into the round groove to finish the bar rolling.

Experimental Method
Study on micro-structure in thi perimental study on small specimen, using low-temperature single-pass large-strain plane-strain compression experiment.Figure 6 is a diagram of plane strain compression experiment.Compared with cylindrical compression experiment, it allows importing greater strain into materials, and there is no "bulging" issue, with more accurate and convenient determination of the stress-strain curve.In the experiment of plane strain compression, the resulting strain concentrates at the fixed end, strain variation range from compressed surface to the fixed surface   Quenching treatment is immediately after compression, en is cut in the compression direction.Cross-section after compression is drawn with a en.This compresthat distribution of comstrain is concentrated at an be observed.
and deformation specim Q345 hot-rolled steel plate, chemical compo ass%) is: C 0.16, Si 0.3, Mn 1.5, P 0.016, S 0.004.Three dimensions after machining are namely length 20 mm×width 10 mm×height 10 mm, direction of compression is perpendicular to the rolling direction, and through thermal simulation testing machine the temperature, strain and strain rate of the specimen can be controlled.Figure 7 is schematic of hot working technology, specimen is heated to the deformation temperature range of 700˚C -500˚C below the Ac1 by 10˚C/s by means of thermal resistance, to make specimen heated evenly, it is insulated 3min in this temperature and then unidirectional compressed where strain rates are 1 s -1 , 0.1 s -1 , 0.01 s −1 respectively, reduction is 80%, and corresponded deformation time are 1.61 s, 16.1 s, 161 s respectively.dotted line in Figure 6, corrosion with nitric acid alcohol after mechanical polishing, and structure observation on the scanning electron microscope.

Result and Discussion
In order to obtain the relationship between microstructure and deformation parameters, first is to clarify the distribution of internal strain in the specim sion method research indicates pressive stress and compressive the bottom of the specimen, that is, the side without deformation, and that distribution of strain in the specimen is continuous.Finite element method is used in this article for numerical simulation of the compression process, and Figure 8 is the result of finite element simulation,  which mainly studies the distribution of strain.From the result of the simulation, there is rigid zone where the indenter and specimen contact, so strain is very little, as larg ness direction of observation area under temperature of pressed surface to the bottom, which indicates that th finite element simulation results become 4 ar le st for large am an provide more nucleation of recrystallization lo rocess, and can be expressed as blue area indicated by the arrow in the picture, while e strain mainly distribut th sides and the bottom es bo of rigid zone.Along the compression direction, strain becomes gradient distribution, with variation range of 0 -4.
Figure 9 is scanning electron micrograph along thick-600˚C and strain rate of 0.1 s -1 , region of deformation belt is the region in red box in Figure 8. From the picture we can know that the deformation belt tapers from the com e large deformation occurs at the bottom, while large strain concentrates at the bottom.The same results were obtained via the same observation of the specimen under other deformation condition.This is the same result with finite element simulation, so it is reliable of numerical distribution of plastic strain obtained by finite element simulation.
Initial microstructure is the layer laminated structure of distribution of ferrite and pearlite, as shown in Figure 10, content of pearlite is relatively little, initial grain size of ferrite is 20 microns, and it is equiaxed distribution.
Figure 11 shows that under different deformation conditions, eas, and microstructures obtained by scanning electron microscope.Pronounced equiaxed grains can be seen from every figure, which indicates that ferrite grain finally formed under these conditions has been in a stab ate and is basically equiaxed grain after quenching.Ferrite grain can be very small by observation, and the smallest grain size is hundreds of nanometers.
Figures 11(a) and (b) are in conditions of 600˚C, and strain rate of 0.1 s -1 and 0.01 s -1 respectively.Through observation of grain size in the picture, it is apparent that grain size is smaller than that in strain rate of 0.01 s -1 after compression in strain rate of 0.1 s -1 .Because under large strain rate, it is more prone to appear ount of strain belts under large strain rate, and defect such as twinning and dislocations, which provide energy and location for nucleation, increase the nucleation rate and lead to grain refinement.Under condition of 600˚C, grain size is less than 1 micron at strain rate of 0.1 s -1 and 0.01 s -1 .
Figures 11(a) and (c) are compression in strain rate of 0.1 s -1 and temperature of 600˚C and 650˚C respectively.Grain size at temperature of 600˚C is apparently less than grain size at 650˚C, and lower temperatures lead to smaller grain size.Because the deformation at low temperature c cations, as well as higher rate of nucleation, and low temperature can effectively reduce growth speed of grain.Under the condition of 650˚C, average grain size is around 1 micron.
In order to illustrate the relationships between grain size and parameters of processing process, the processing parameter Z is introduced, which is an important parameter that comprehensively describes deformation temperature, strain and effects of strain rate on deformation characteristics in p where  is strain rate; Q is surface deformation activation energy dition, Q = 254 kJ/mol); R is gas const ; T is a deformation teme.
the grain size after recrystallization is taken av ant.Average grain sizes of several ferent conditions of temperature an Z-value is calculated under co relationship between dynamic recrystallization grain size and Z valu Judging from the relationship bet Z nder large strain, la (independent from the stress con ant peratur In dynamic recrystallization process, each grain is at different stages of recrystallization, so grain size is different, and erage grain size.Its average grain size is related to d and Z values, it can be determined according to the following relationship: where A, m are const specimens under difd strain rate are measured in this article, while rresponding conditions, according to the iterative analysis in Formula (2), relationship between grain size and Z value is as follows: Figure 12 is curve es.ween grain size d and value, grain size gradually decreases as the Z value increases.At low temperature and u rge strain rate is needed to access small grain size, however, when the Z value increases to a certain extent, it is not obvious that grain size decreases as the Z value increases.From the perspective of manufacturing ultra-fine grain, Figure 12

value, at th
e o e tim f metal plastic processing, such asrolling, forging, important deformation and machining parameters, such as strain, strain rate and temperature, can be obtained though certain grain size in metal processing.Desired grain size can be achieved by controlling these parameters.If grain of 1 micron is needed in the engineering application, the desired Z-value can be obtained to be 2.17 × 10 13 through Figure 12.
Relationship between the deformation par ameter can be obtained by Equation ( 1), according to the requirements of the designers, the control of grain size can be ultimately realized by controlling deformation process parameters such as strain, strain rate and temperature.According to new flat-oval-square groove in the earlier design, using simulation of rolling process by finite element software, the cross section shape and strain distribution after each pass rolling of flat-oval groove are shown in Figure 13, where the equivalent plastic strain curves were distribution of rolled piece section of arrow position from left to right.The centre equivalent plastic strain of these rolled piece cross sections were 0. 90, 4.20, 4.75, 5.25 respectively.These cross sections strain distribution cloud pictures show that the strain was gradually introduced into the inside cross section and the largest accumulation of strain was more than 5.0.Strain value at the center of section in the traditional mond-square groove and square-section rolling i and large on the boundary, and new flat-oval gr large strain in the center and gradient of strain distri tion changes small.And large plastic strain in the h helps control heart defect, which leads to efficient compaction in the center.

Finite Element Simulation Results
To make the ultra-fine grain bar, not only the strain should be considered but also the temperature change of rolling process.Generally speaking in the process of rolling temperature changes were caused by three aspects: 1) Plastic work generated heat during the rolled piece deformation.2) Frictional heat generated between rolled piece and mill roll.3) Rolled piece had heat transfer itself, including contact heat transfer between rolled piece and mill roll, rolled piece of radiation and convection heat transfer.Compared with plate rolling, result in the temperature was hard to predict in bar rolling because the rolled piece deformation was not uniform which was influenced by the groove.Therefore, it was necessary to compare the numerical simulations and experiments, and then repeatedly revised the parameters of model.Figure 14 shows the rolled piece surface temperature distribution of each pass after rolling.
From temperature change of exit of the groove system workpiece, there is little difference between temperature changes of each pass, temperature rises of workpiece during the previous four times are large, because large deformation leads to high heat by plastic work.The latter two passes are mainly control of the shape and size of workpiece, the deformation is relatively small, so the temperature is lower than the first few passes.

Prediction Simulation of Microstructure
Microstructure prediction method used here is VUSD-FLD subroutine of ABAQUS, which is open program interface ABAQUS provided for users, new output variable that user definite under dynamic conditions at the material point, and user writes subroutine to implement the required functions.Generally FORTRAN language is used as its compile language.Its code interface is: subroutine vusdfld( c Read only variables - the analysis result of ABAQUS with calculation formula of grain size its progr gr nd its forecast form is shown in Figure 15 5. e all section obtained bigger strain.between grain size and value of Z c am of system, a c Local arrays from vgetvrm are dimensioned to .c maximum block size (maxblk) Above cloud of grain forecast indicates that ultra-fine grain has been introduced to the entire section eventually and grain sizes are all smaller than 1 micron.The Figure shows grain size in the center of the bar is larger than grain size around because in the rolling process, the strain in the center is large, the temperature is high caused by more plastic work heat, which matches the

Figure 1 .
Figure 1.The model of flat-oval/square caliber series.

Figure 4 .
Figure 4.The shape of three calibers and strain distribution situation.

Figure 5 .
Figure 5.The new grooves of bar rolling.

Figure 6 .
Figure 6.Schematic plot of plane strain compression.grain size under different strains c Materials used in this experiment are part cutting from sition (m

Figure 8 .
Figure 8. Simulation result of single direction compression.

Figure 11 .Figure 12 .
Figure 12 is curve es.ween grain size d and value, grain size gradually decreases as the Z value increases.At low temperature and urge strain rate is needed to access small grain size, however, when the Z value increases to a certain extent, it is not obvious that grain size decreases as the Z value increases.From the perspective of manufacturing ultra-fine grain, Figure12also has a certain engineering