Role of Heating Conditions on Microcrack

Objectives:

•  To investigate the range of material property variation observed in conventional roll forming and how it affects part shape quality and specific shape defects like twist, flare, and bow.
•  To separately understand the effect of residual stress and microstructure on roll forming process robustness and component shape.
•  Based on the above to develop and experimentally validate techniques for material monitoring and part shape control.

Introduction:

Roll forming is one of the most adaptable forming processes, efficient enough to produce extended cross sections by passing a strip of steel material through consecutive pairs of rolls (10). It is extensively used in the automobile industry to produce components that are lightweight and increased material strength which results in greater vehicle safety, reduced greenhouse gas emissions, and improved fuel economy.

Conventional roll forming application is limited in the application of the automotive industry because this process is confined to longitudinal components with a constant cross-section. But the automotive parts generally have variation in shape and A new sheet forming process is flexible roll forming which can support to manufacture components with a varying cross-section.in the process of flexible roll forming the form rolls are free to go around the edge profiles of a pre-cut sheet. Additional Two degrees of freedom transitional and rotational are set in motion which allows forming parts with a non-uniform cross-section. This leads to difficulty in forming contact state in numerical analysis results in higher computational costs (5)

Ultra high strength steels(UHSS) have high strength up to 1500 to 2000 MPa. They are developed to manufacture complex parts with extended safety. They are coated with AL-Si to offer corrosion resistance or coated with Zn to provide active cathodic production(12). High strength low alloy (HSLA) steels, especially dual-phase (DP) steels are extensively used in the automobile industry since they have unique properties as good ductility, high tensile strength, low yield strength ratio. DP steels are well known for crash resistance, good formability, and superior surface finish (9). The hardness measurements indicate that the DP steel surface hardness is affected while the plastic deformation process.

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In engineering the structure of low-density steels, it is essential to verify the properties of discrete ordered phases and their mechanical properties of samples at room temperature which vary with the change in precipitates composition. High Al low-density steels have a transformational effect in transportation on lightweight structures because of their superior properties like resistance to corrosion and oxidation they are used to replace stainless steel. But alloying with Al there is a significant reduction in mass density therefore they are favorable for the automotive body (13).

Purpose and Aims

The microstructural and micromechanical study is necessary on DP steels because the microstructure of steel can undergo serious damage during roll forming. The outer surface is highly deformed which results in the growth of voids or cracks and the inner surface undergoes compression. This develops a localized plastic strain at the outer and inner regions of the bent corner. This has a significant influence on fatigue strength. Therefore, further research and mechanical testing should be performed to understand the microstructure’s influence on mechanical properties.

In the rolling process while the running speed is increased it leads to vibration problems. The cold strip mill production can be increased if the variation in thickness of the aluminum strip which is hot rolled can be lowered. New challenges were faced due to demands on torques and tolerable rolling forces simultaneously the design of roll pass has a dominant effect on product shape, power consumption, and gage variations. The significant contribution from the previous study was done on the precision analysis of variation in torque and roll force during the rolling process. the results from the experimental data showed that torque values and rolling force are not constant. It has been predicted from results with the rise in rolling speed the fluctuations in torque and rolling force is increased. But the relation on the dependence of these parameters on the process is not clear and further research is required (7).

Residual stress has a vital role in designing steel structural members. Residual stresses in cold-formed sections are usually caused by roll forming. In compressed members depending on orientation, magnitude, and distribution of these stresses can be favourable or unfavourable Residual stresses are caused due to Phase transformation, temperature variations, and Mechanical treatments (bending, rolling, extruding, and tensile testing). Controlling residual stress is very important as it cannot be avoided for the components exposed to fatigue or stress corrosion cracking conditions. Several mechanical treatments such as shot peening, light cold rolling, and laser peening are used to induce compressive residual stress. Measuring residual stress can be done through a non-destructive X-ray diffraction technique. The information from residual stress data can improve the model accuracy in finite element simulation to capture shape defects in the roll forming process.

High quality of the product is essential to meet the standards and this can be achieved by increasing material strength which affects the final component shape due to variation in the material property. The metal strip used in roll-forming is pre-processed by skin pass rolling or tension levelled for improving flatness. This might induce residual stress which affects the part shape. Therefore, reliable methods should be developed allowing safe, efficient products and achieve process robustness. Advancement in existing techniques and new methods should be developed for inline property monitoring and part shape control. (1)

Microstructure, depth of cracks, morphology were analysed using techniques like Scanning Electron Microscopy, Conventional optical microscope, Focussed ion beam imaging, and Energy dispersive spectroscopy. Steels coated with zinc offer ultra-high-strength steels with corrosion resistance. Hot forming causes zinc to infiltrate grain boundaries which leads to intergranular cracking of the base metal. Generally, heating conditions do not have a major effect on the spatial distribution of cracking on top and sidewalls.

But there is a clear indication that penetration depth and cracks penetration is reduced into the martensite prior austenite with an increase in time of heating and temperature. A complete study has to be done further to completely understand the effect of heating in zinc-coated 22MnB5 on microcrack formation (11). The reason for cracking in Zn steel is a combination of various reasons. Cracks initiated can be a result of the varying coefficient of thermal expansion of coating and substrate and the other reason can be microcracks initiated on the surface due to friction(12)

On-Going Research

To increase the efficiency and accuracy of shape prediction process, a simple bending test has been developed at Deakin University with a great deal of complexity which can be used for thin strip bending to high bending strain, which can be used to monitor variation in material properties from the coil to coil allowing a great accuracy in measurement near to material yield and low forming strains. Further, this method involves a combination of numerical analysis and simple inverse routine which gives the information about residual stress profiles in the incoming strip. This data can be used in FEA to predict material behaviour. (1)

FEA allows to lead time and also improve the cost efficiency of the design. There is a better understanding from the data of stress-strain attained by the inverse method from the bend test as input to FEM comparatively than the material data acquired from the tensile test. Due to the unavailability of details regarding strip processing is might not be possible to get detailed residual stress information and besides analysing metal processing data is time-consuming. A combination of bend test data of material input and assuming the homogeneous material behaviour is used in this study. Which accounted as a cost-effective substitute for determining the effect of residual stress in the numerical analysis of the roll forming process. (2)

A newly developed approach for in-line shape control is by utilizing roll load, torque measurements and with a combination of FEA analysis are used to identify variation in properties and relate it to the final part shape. From the current results, it can be shown that material data from an inverse routine can be used for simulations of roll forming with sensible model precision. the inverse routine must be modified further as discrepancies are observed between FEA predictions and experimental results. thus, further study is required to demonstrate the solid shell element functionality.

Reduction in thickness is done in the stress relieved material during the rolling process in demand of lightweight structures which induces tensile residual stresses at the surface of a material and the middle of sheet is affected by compressive residual stress (1). From experimental and numerical results, it is observed that due to reduction in thickness the final shape of a product is affected and if residual stress is present tensile testing is not enough to have a precise understanding of shape defects.

Material which is isotropic hardened was used so far for the rolling process to reduce thickness. FEA while considering a material model which is kinematic hardening might change the profile of final residual stress. assuming isotropic hardening material model is appropriate for bend test, a reversal in load might occur during the bend test while using thickness reduced material.it is suggested from the past conducted research which is better to apply kinematic or combined hardening (4). It has also been observed that not only the thickness reduction that effects residual stress but also the number of skin passing steps used have a crucial effect (6).

To extend and enable the roll forming process application in the automotive industry new techniques should be developed to recompense the shape defects without any intervention in the roll forming line. monitoring the process parameters such as roll load and torque in the inline shape compensation method is required.in addition it is essential to know the parameters that relate the variation in the properties of the material on the final part shape to adjust the tooling accordingly or other techniques compensate the shape defects. this might enable the inline compensation of shape defects for future roll forming lines (3).

A combination of numerical, theoretical, and experimental techniques were proven laboratory environments but the residual stress effect induced due to reduction in thickness on part shape needs further research and development. besides the effect of variation in material strength on part shape residual stress has a major influence. The utmost concern for using an analytical approach to estimate residual stress in a metal strip is due to insufficiency of accurate details concerning the pre-processing state of the material incoming.

References

1.  The influence of residual stress on a roll forming process . A. Abvabi, B. Rolfe, P.D. Hodgson, M. Weiss. 2015, International Journal of Mechanical Sciences.
2.  Effect of coil set on shape defects in roll forming steel strip. Matthias Weiss, Buddhika abeyratna,Bernald Rolfe,andre Abee.Henry Wolfkamp. 2016, Journal of Manufacturing Processes.
3.  An analytical approach to predict web-warping and longitudinal strain in flexible roll formed sections of variable width. Jingsi Jiao, Bernard Rolfe, Joseba Mendiguren, Matthias Weiss. 2014, International Journal of Mechanical Sciences.
4.  The effect of skin passing on the material behavior of metal strip in pure bending and tension. Matthias Weiss, Will Ryan, Bernard Rolfe and Chunhui Yang. 2010. NUMIFORM 2010 :Proceedings of the 10th International Conference on Numerical Methods in Industrial Forming Processes,American Institute of Physics (API).
5.  An investigation on the roll force and torque fluctuations during hot strip rolling process. Bisadi, Mahdi Bagheripoor & Hosein. 2014, Production & Manufacturing Research .
6.  A first step towards a simple in-line shape compensation routine for the roll forming of high strength stee. Matthias Weiss, Buddhika Abeyrathna , Bernard Rolfe , Peter Hodgson. 2015, THEMATIC ISSUE: FLEXIBLE FORMING – INCREMENTAL SHEET FORMING & ROLL FORMING.
7.  Abvabi, Akbar. Effect of Residual stress in roll forming of metal sheets. s.l. : Deakin University, 2014.
8.  Reduction of Anisotropy in Cold-Rolled Duplex Stainless Steel Sheets by Using Sigma Phase Transformation. G. FARGAS, N. AKDUT, M. ANGLADA, and A. MATEO. 2011, The Minerals, Metals & Materials Society and ASM International.
9.  Microstructural and Micromechanical Effects of Cold Roll-forming on High Strength Dual Phase Steels. Meritxell Ruiz-Andresa, Ana Conde,Juan de Damborenea,Ignacio Garcia. Print version ISSN 1516-1439On-line version ISSN 1980-5373, Centro Nacional de Investigaciones Metalúrgicas – CENIM-CSIC, Madrid, Spain : Materials Research Ibero-american Journal of Materials , 2015.
10. Flange Wrinkling in Flexible Roll Forming Process. Mohammad Mehdi Kasaei, Hassan Moslemi Naeini, Behnam Abbaszadeh ,Mehran Mohammadi, Mojtaba Ghodsi, Manabu Kiuchi, Reza Zolghadr, Gholamhosein Liaghat, Rohollah Azizi Tafti ,Mehdi Salmani Tehranie. s.l. : Procedia Engineering, 2014.
11. ROLE OF HEATING CONDITIONS ON MICROCRACK FORMATION IN ZINC COATED 22MnB5-2014. Janik, Vit, et al. 2014.
12.  Zn Diffusion and [alpha]-Fe(Zn) Layer Growth During Annealing of Zn-Coated B Steel. VIT JANIK, YONGJUN LAN, PETER BEENTJES, DAVID NORMAN,GUIDO HENSEN, and SEETHARAMAN SRIDHAR. 2015 : The Minerals, Metals & Materials Society and ASM International.
13. Nano-mechanical properties of FeMn-Al-C lightweight steels. Alireza Rahnama, Hiren Kotadia, Samuel Clark, Vit Janik & Seetharaman Sridhar. s.l. : Scientific Reports, 2018.2