WO2013164847A2

WO2013164847A2 - A method to reduce end defects in rolling of sections

Info

Publication number: WO2013164847A2
Application number: PCT/IN2013/000180
Authority: WO
Inventors: N. Mahadik K.; P. Date P.; Shashikant Nalawade Rahul; Satish K Cheekatla V.
Original assignee: Indian Institute Of Technology Bombay; Bharat Forge Limited
Priority date: 2012-03-19
Filing date: 2013-03-19
Publication date: 2013-11-07
Also published as: WO2013164847A3

Abstract

The present invention aims at reducing the cropping losses formed during reversible multi pass hot rolling of flat ended bloom into bars by designing the preform end shapes using finite element analysis and a method to provide the designed preform end shapes to the input blooms. A three dimensional simulation of the reversible multi pass hot rolling process with flat ended input bloom predicts the formation of defective end shapes after rolling and reverse material tracking is applied to finite element simulations to obtain desired preform end shapes of the input bloom and similar simulations on the bloom with preformed end shape shows considerable reduction in cropping losses. An open die forging is used to provide the preform end shape to the input blooms obtained by reverse material tracking method and rolling of the said input blooms by ordinary rolling process shows considerable reduction in the formation of cropping loss and exhibits an effective improvement of about 1.02% in the yield of rolling process.

Description

Title

A method to reduce end defects in rolling of sections Field of the Invention

The present invention relates to a novel method of reducing cropping losses occurred due to formation of fishtail and overlap during hot rolling of the blooms into bars. More specifically, it relates to a process of reversible multi pass hot rolling of flat ended bloom to reduce the end defects formed during rolling.

Background of the Invention

The steel industries today are focusing on increasing the yield in the metal forming processes, which are affected by the steel loss incurred depending on the process route, size and shape of the filial product etc. Most of the steel blooms are cast in steel works then charged into soaking pits as in a hot state and rolled to provide bars of different cross-section like round or square. During the hot rolling of the blooms, the end of the bloom is distorted due to the rolling action and concave shape is formed. This is observed at either end of the bar, along the thickness as well as along the width. These end shapes are not desirable in finished bar, as they must be cut and discarded as scrap to provide a sound finished product. The end defects are 'Fishtail and overlap', which are the natural result of rolling action. These defects are produced due to non-uniform deformation in the work piece which is produced due to different material flow in the surface layer and at center that is, metal flow in the surface layer becomes higher than the central portion of the material which produces concavity at the ends.

Many earlier researchers found that by providing a proper end shape to the input material it is possible to reduce these end defects. The influence of variation of width and thickness reduction on cropping loss in ingot rolling was studied. It was observed that by increasing thickness reduction, the overlap component of cropping loss increases but has little effect on fishtail component. On the other hand increasing the width reduction increases the fishtail component with little effect in overlap. Researchers also considered truncated pyramidal shape at bottom of the ingot which reduces the cropping loss. Besides there have been trials conducted by industries to reduce the amount of overlap by providing the taper end shape. US 4344309 discloses a process for preventing growth of fishtails during slabbing and more particularly to a method of reducing cropping loss caused by the growth of fishtails produced at longitudinally opposite end portions of the top and bottom of a material to be rolled during slabbing. US 6453712 relates to a method and apparatus for reducing cropping losses during slab and ingot rolling by giving a specially configured shape at the ends of the ingot. The shape is provided by specially shaped bottom block or starter block during casting. US 4387586 provides the methods and apparatus for rolling rolled materials width wise thereof wherein a slab of metal having a large width as contrasted with the thickness is rolled width wise thereby formation of fishtails at the lengthwise ends of the slab are avoided. Most of the prior arts apply to steel and cast iron. None of the above mentioned prior arts specifies a methodology to determine the end shape of the input bloom and achieve the flat ends consistently after rolling due to feeding of convex shaped bloom end preform to rolling. The method of end pressing between flat platens to generate the convex shape is also not recommended by any of the above mentioned prior arts. The present invention, being a method, a process, is applicable to all metallic metals and alloys being rolled. Regardless of whether the billet is produced by ingot casting or continuous casting or any other method, the present invention applies with equal facility.

In the present invention, it is practically possible to achieve the flat ends consistently after rolling due to feeding of convex shape bloom end preform to rolling thereby reducing scrap loss due to fishtail formation. It is feasible to use conventional rolling mills which are economical for high productivity at lower cost by using the process used in the present invention. In the present invention, it is practically possible to roll the bloom in one heat. Usually the grain compaction is found to be better with end pressing than recess formation through reversible rolls at bloom ends. In view of the foregoing it is thus established that by providing proper end shape to the input material, it is possible to reduce the cropping loss. Hence there is a pressing need to provide a proper end shape to the input material in order to reduce the end defects formed during rolling, thereby minimizing the cropping loss. Thus the present invention aims to determine a preform end shape which gives flat ends after rolling resulting into a reduction in cropping loss thereby leading to substantial amount of material as well as monetary savings.

Objects of the Invention 1. It is the primary object of the present invention to provide a novel, simple and economic method of reducing cropping losses due to formation of fishtail and overlap that occurs at the ends of the rolled bar during reversible multi pass hot rolling of the blooms into bars. 2. It is another object of the present invention to provide a proper end shape to the input material of rolled bar. 3. It is another object of the present invention to reduce end defects and scrap loss of the rolling bar thus, obtaining flat end shape in finished bar after rolling and leading to substantial amount of material as well as monetary savings.

Summary of the Invention The present invention aims at determining the preform end shape of the bloom which gives flat ends after rolling, thus minimizing the end defects and cropping losses in the rolled bar.

The process employed in the present invention is accomplished by designing and manufacturing convex shape bloom end preform to rolling using reverse material flow technique (also called as back tracing method) and by pressing the two ends of the work piece along thickness wise and width wise directions respectively that helps generate the desired bulge at either end and leads to the consistent flat ends in the finished bar after rolling.

Brief description of the drawings

Fig. 1(a) shows the conventional rolling process. Here the flat ended input bloom is rolled with multipass rolling operation in to the finished bar. The defective end shapes are shown.

Fig. 1(b) shows the method of marking on the billet prior to deformation. Fig. 2(a) shows the location of the strip relative to the defective end shapes in a simulation result for reverse material tracking technique to obtain preform end shape of the input bloom.

Fig. 2(b) shows the deformed position of the straight line marks made on the billet normal to the direction of rolling.

Fig. 2(c) shows the method of locating points on a single plane for reverse material tracking technique to obtain the bulge profile before forming and planar profile after forming.

Fig. 2(d) shows the back traced shape of the bloom at the two ends, determined by simulation.

Fig. 3 shows the convex bulge formation due to the lateral material flow when pressed between two flat dies.

Fig. 4 shows preform end shape formation with end pressing of bloom up to particular depth and length with flat dies in thickness as well as in width wise direction.

Fig. 5 shows the invented manufacturing method adopted to reduce the end cropping loss formation after the rolling. Detailed description of the Invention

Generally, in conventional rolling process, a billet produced by continuous casting has flat end shapes. This input material is rolled in both thickness wise as well as in width wise directions by rotating the billet by 90 degrees during rolling to form the final bar. Upon such rolling, material ends get distorted by the continuous rolling action and forms defective ends, which cause the cropping loss of about 3- 5 percent of the total bloom weight.

The present invention proposes a method for reducing cropping loss by employing two steps namely (A) Designing the preform end shape of the input bloom (B) A method to provide preform end shape to the input bloom.

(A) Designing the preform end shape of input bloom In the reversible multi pass hot rolling process, the end defects are formed due to non-uniform deformation of the material. Providing proper preform end shapes can make the metal flow at the ends more uniform during rolling and reduce the tendency to form end defects. In the present invention, the design of preform end shape can be carried out by deploying the back tracing method either by using a computer simulation or using the rolled bar in the shop.

Designing the preform end shape using computer simulation can be carried out in two steps. Step-1: Simulation of conventional rolling process:

As a first step of designing a preform end shape for the input bloom, a three dimensional simulation of hot reversible multi pass rolling process is carried out for the conventional rolling process using actual input parameters from rolling mill. In reversible multi pass hot rolling process, input bloom is converted into output by passing it between grooved rollers of different groove widths. The bar being rolled undergoes certain forward and reverse passes. In the process of rolling, the concavity formed in first pass elongates in each subsequent pass. Front end concavity and back end concavity develops over the several stages during rolling leading to fishtail defects (Fig. 1(a)). Step-2: Reverse material tracking simulation:

The reverse material tracking simulation is carried out to determine the preform end shape of input bloom. In reverse material tracking, back grain flow tracking of material determines the initial position of the deformed nodes (after rolling) in the input bloom. The final bar formed after rolling with defective ends is superimposed with flat strips at the ends as shown in Fig. 2a to identify the nodes falling within these strips. Pursuant to identification of initial position of the nodes in the bloom, the strips are positioned at both front end and back end of the rolling bar. Then reverse material tracking is carried out from final bar to initial bloom before first pass through all loading passes which were used during rolling. The front end shape and back end shape obtained after the back flow gives convex contour at both ends as shown in Fig.2d. The preform end shape imparted to input bloom forms less end defects on rolling than that obtained after flat ended bloom rolling. With a few iterations, the final optimized convex shape at the ends of the billet is obtained.

Designing the preform end shape using the rolled bar in the shop.

Whatever is accomplished by simulation can be accomplished on the actual rolled bar in the shop using the steps enumerated as under:

a. Make indelible line marks parallel and perpendicular to the billet length at certain constant intervals on the undeformed billet at two ends upto a certain distance from the ends (calculated depending on the maximum length of the observed defect), Fig. 1 (b). b. Deform the billet to the final cross section. This deformed product will have the defect that needs to be corrected. The indelible marks would have moved to new locations as shown in Fig. 2(b). c. Locate points of intersection between the deformed straight line contour as shown in Fig.2(b) with lines drawn normal to the direction of rolling (Fig.2(c)). Use these points for determining their corresponding locations on the undeformed strip. In case of a computer simulation, mark two strips at the two ends before forming, such that they occur inward of the defect after forming as in Fig.2(a).

Back trace the location of the nodes in the strips marked (Fig. 2(a)) to their corresponding locations on the original billet in case of simulations (Fig. 2d). The points of intersection determined on the billet (Fig.2(c)) are similarly back traced to their corresponding locations on the billet to obtain the desired shape.

By compressing the ends of an undeformed billet between flat platens, the end profiles are obtained as shown in Fig. 3. Thickness wise and width wise reductions during end pressing are illustrated in Fig. 4.

Roll the billet as usual and if desired, iterate through steps to improve on the yield (Fig.5).

Table 1 gives details of improvement achieved after few iterations of the above simulation based procedure having an effective improvement of about 1.02% in the yield, which amounts to savings of about Rs. 12-20 lakhs per month for a medium scale industry. Table 1 Comparison of end crop loss after rolling of bloom with flat end and preform end shape in terms of weight percentage of total bloom weight (Simulation results)

(B) A method to provide perform end shape to input bloom

The preform end shape so obtained by the above process is imparted either by machining or forging or adopting new mould design in actual practice. In the present invention, forging is employed as it reduces the material loss. As shown in Fig. 3, when material is being compressed between two flat dies, a dead metal zone forms below the die surface where no strain occurs, which causes the material to flow in axial direction from the non-deforming part of the work piece and forms the convex bulge at the ends. In the present invention, a preform end shape is given by pressing the ends of the bloom using open die forging to predetermined lengths and depths in both thickness wise and width wise directions as shown in Fig. 4. Initially the top and bottom portions of the bloom are pressed in the thickness direction between two flat dies by thickness reduction of ΔΤ up to certain length AL. Then bloom is rotated through 90° and top and bottom portions of the ends of bloom are pressed in the width wise direction by reduction of AW. It is because of the end pressing operation that the end convex bulge is formed at both ends of blooms. Due to the end pressing operation, a step at the ends of the bloom with reduced cross section is formed. This end step helps to reduce the end defect formation, because the deformation caused during rolling for first few passes is not transferred to the ends of the bloom. Fig.5 shows the invented manufacturing method for obtaining flat ended final bar after rolling. In this method, rolling of the preform end shaped input material so obtained is carried out with regular rolling pass schedule.

The said method is applicable to different sizes of bloom with different rolling schedules. The present invention thus improves material recovery by reducing end cropping losses and increase rolling mill productivity in rolling of sections.

As described above, forging of the ends of the bloom is done at both the ends in thickness wise as well as in width wise directions and thereafter ordinary rolling is effected on bloom, so that fishtail can be prevented from growing, thereby enabling to obtain an outstanding advantage in the reduction of crop loss. The experiments are carried out to optimize the width to thickness reduction ratio for effectively preventing fishtail from growing. The trials are conducted by varying the number of strokes of forging i.e. 2 strokes (2 strokes: Pressed in width wise direction in single stroke (1^st stroke), then rotation through 90° and again pressed in thickness wise direction in single stroke ( 2^nd stroke) and 4 strokes (4 strokes: Pressed in width wise direction in single stroke (1^st stroke), then rotation through 90°, pressed in thickness wise direction in single stroke (2^nd stroke), rotation through 90°, again pressed in width wise direction in single stroke (3^rd stroke), then rotation through 90° and again pressed in thickness wise direction in single stroke (4^th stroke). Also trials are conducted by varying width to thickness reduction ratio from 0.3 to 0.6.

Upon studying the effective combinations of various trials, it becomes evident that the end cropping can be possibly reduced by maintaining width to thickness reduction ratio between 0.3 to 0.5 and having four number of forging strokes. Width to thickness reduction ratios more than 0.5 results in large crop loss with final convex end shape whereas, the reduction ratio below 0.2 results in concave end shapes of final rolled bar and hence, increase in crop loss.

Therefore, in the present invention, AW/ΔΤ is restricted in the range varying from 0.3 to 0.5.

Case study

The following example illustrates the application of the procedure as per the present invention to actual deformation of a billet by rolling and the consequent improvement in the cropping loss. In this example the bulge profiles at either end followed by end pressing along the length and the width to obtain the bulge profile were established iteratively using an FEM simulation. Thereafter rolling was carried out and considerable reduction in the cropping loss was obtained at the end of the process. EXAMPLE

The rolling yield with the conventional method and with the invented rolling method is compared with each other in producing a finished bar of 160 mm round corner square (RCS) and approximately 17000 mm length from the input steel bloom of 38MnVS6 material with 320 mm (W) by 400 mm (T) cross-section and 3480 mm length. The weight of steel bloom was near about 3.5 ton.

The following was the conventional rolling method:

The hot reversible multi pass rolling was carried out for input bloom with flat ends. The grooved rollers were used for rolling, which consist of grooves of different width and different depths. A typical pass schedule of total 15 passes consists of forward and reverse passes used for carrying out rolling. The bloom passes through different grooves of rollers to reduce its cross-section. During the rolling the cross-section of bloom is reduced in both directions i.e. in thickness (T) as well as in width wise (W) direction by rotating the bloom by 90° rotation by manipulator at several passes repeatedly, thereby producing bar of having cross-section of 160mm round corner square.

Table 2 shows the rolling yield, crop loss and scrap loss obtained through above mentioned method

The method according to the present invention was carried out as described below: In the present invention, before rolling of the bloom, it is provided with the preform end shapes by the open die forging method. The preform convex bulge and consequent thickness reduction (ΔΤ), width reduction (AW), and pressing length (AL) was decided by carrying out the end forging simulation. This was provided with open die forging. Dies selected for end pressing operation have 400 mm width, so end pressing length was assumed as 300 mm i.e. ¾^th of the die width. The thickness and width reductions during end pressing operation were decided using the condition that the strain should reach at the centre. In simulation it was seen that strain produced with the reduction of 120 mm in thickness direction reaches up to the centre, so this reduction was given in thickness direction i.e. ΔΤ. Simulation was then carried out for end forging operation for various width reductions AW to obtain optimum AW /ΔΤ required for convex bulge depth. The optimum value obtained by simulation for AW /AT was 0.5.

The actual shop floor trial was carried out with the same parameters AW, AT, AL from simulation results. The obtained preform end shape with this trail matches well with the simulated one.

The further rolling of this preform end shaped bloom with similar pass schedule as that of conventional one was then carried out to obtain the final bar of 160 mm RCS.

Table 3 shows the comparison between the results obtained with application of present invention and with conventional method.

Claims

We Claim:

1. A method of reducing end defects during rolling of blooms comprising:

a. designing a preform end shape of an input bloom to be rolled,

wherein the preform end shape is designed using back tracing method; and b. providing the designed preform end shape to the input bloom.

2. A method as claimed in claim 1, wherein designing the preform end shape comprises the steps:

a. marking indelible marks parallel and perpendicular to the bloom length at defined intervals;

b. rolling the bloom with said indelible marks to let the bloom undergo end deformation;

c. locating points of intersection between deformed contours with lines drawn normal to the direction of rolling;

d. back tracing said points of intersection to their corresponding locations on the bloom to obtain the desired preform end shape; and

e. determining the preform end shape.

The method as claimed in claim 2, wherein the preform end shape is a convex shape.

The method as claimed in claim 1 , wherein the preform end shape is provided to the input bloom using open die forging.

5. A method as claimed in claim 4, wherein forming of the preform end shape to the input bloom comprises the steps:

a. pressing top and bottom portions of the ends of the bloom between two flat dies in width wise direction by width reduction of AW; and b. rotating the bloom through 90° and pressing top and bottom portions of the ends of the bloom between two flat dies in thickness wise direction by thickness reduction of AT.

6. The method as claimed in claim 5, wherein width to thickness reduction ratio is in the range varying from 0.3 to 0.5.

7. The method as claimed in claim 5, wherein there are plurality of forging strokes.

Dated this 19^m day of March 2013

(Authorized Agent for the Applicants)