Sanat Kumar Bhaumik a first class mechanical engineering graduate from Bengal Engineering College, Shibpur, University of Calcutta with MBA from All India Management Association, New Delhi has more than 37 years of total experience in MECON Limited, Wean United Inc., Siemens VAI Metals Technologies and Danieli India in the areas of design, technology, proposal engineering, project management, erection and commissioning of Mills / Lines for Flat Product Casting, Rolling & Processing. Widely traveled and much sought after as an expert across domains he is the Senior Vice President (Flat Products) in Daneili India, spearheading the march of the Italian conglomerate.
HSPM : AN INNOVATIVE TOOL TO ADD VALUE FOR YOUR HR COILS – Sanat Bhaumik, Sr. Vice President (Flat Products) of DANIELI India
Today’s demanding market is pushing steel strip processors to a new height. The requirements for steel strips are of reliable consistent quality at competitive price. This market demand leads to development of newer grades of material at competitive cost. To minimize the overall cost of production for the end users, where ever possible they would prefer to use hot rolled coils over cold rolled coils. This necessitates production of hot rolled coils with tighter tolerances and much better surface finish. For example, the use of hot rolled strip for pressed components and tube making has initiated requirement of good shape, yield point suppression and specific texture. In the hot rolling process it is not possible to control these parameters due to the fact that rolling takes place at higher temperatures. The end products from hot skin passing are produced at ambient temperature where the properties of material are different than at elevated temperature.
Core Communique spoke to Sanat Bhaumik, Sr. Vice President (Flat Products) of DANIELI India to understand the benefits of hot skin passing technology and its relevance in India steel industry.
Q1: First of all please tell us about this new technology.
A1 : Hot skin passing operation is performed at room temperature to improve the strip quality with respect to flatness, elongation and surface roughness of the hot rolled coils produced from a conventional hot strip mill or thin slab and casting & rolling plants. Such hot skin pass mills also reduces the rejection level of hot rolled coils by the very demanding customers as well as it can also be used as coil dividing lines to meet the specific requirements of customers. In this operation hot rolled strip is passed through a set of rolls to get 1% to 3% elongation. The input strip can be black hot rolled (HR) or hot rolled pickled and oiled (HRPO) depending upon the end products requirement. Most of the Skin-pass mills are standalone installations. However, Hot SPM in-line with pickling line is also widely used for specific applications. The stand alone mills are more flexible in operation as compared to inline ones.
Hot skin passing can be dry as well as wet type. The wet type can provide elongation more than 3%. While the dry type is equipped with a Scale/Dust Extraction system, the wet type mills have Fume Exhaust system and sludge removal facility.
Q2: What are the main purposes for the Hot Skin Pass mills?
A2 : The main purposes of the hot skin-passing operation are –
1. Developing appropriate mechanical properties
2. Improving strip flatness
3. Transferring roughness at strip surfaces, for pickled material
4. Improving shape of Coil Winding
5. Recoiling and dividing the coil
Q3: What are Luder Lines and how they can be eliminated by hot skin passing operation?
A3 : During metal forming operation, rough surface with stretch marks are observed. These stretch marks are called as Luder Lines. The stretch marks are caused due to uneven permanent deformations. At the yield point the material exhibits increase in the strain with slight drop in the stress value exhibiting permanent plastic deformation. Loading the specimen beyond yield point, the material again shows increase in the strength with increase in strain. Due to the elongation, the free atoms of carbon and nitrogen leave the dislocations inside the grain structure. Dislocations are free to move and further deformation (i.e. stamping) is more uniform. If the tensile test is carried out on skin-passed (cold worked) material, the stress strain curve does not show distinctly the yield point and there is gradual change of slope in the transition zone. This shows that a little cold working suppresses the distinct yield point phenomena.
During skin-passing the material is stressed beyond yield point to achieve permanent deformation. Luder’s lines are not generated in stamped strip which was previously skin-passed, within a certain time limit. If the material is not deformed within a certain time after skin passing, yield point will occur again (material aging) and consequently the Luder’s lines after the following deformation. This storage time depends upon room temperature, the higher the storage temperature the shorter the storage time; material aging is typical for common carbon steel. The Skin-passing is essential for mild steel and ferritic grades of stainless steel. It may be noted that for material with the higher yield strength and thickness, there may be increase in elongation of the strip (0.5 % – 3.0%). Also due to elongation of strip there is improvement in the shape.
Q4: How do the HSPMs improve the strip flatness of HR coils?
A4 : During hot skin-passing operation, there is increase strip flatness due to the fact that the strip is elongated in the rolling direction under controlled tension conditions, so that all fibers are of the same length leading to flat strip. To achieve controlled elongation a constant front and back tension is maintained. The work rolls in the skin-pass mill are provided with bending facility to facilitate generation of roll crown. Waves defects can be corrected using positive and negative bending and suitable work roll crown. These skin-pass mills can be provided with anti crimping roll (effective under 1.5mm strip thickness) and these rolls help in eliminating the cross bow in the strip.
Q5: You have also mentioned about “Roughness transfer”…
A5 : The hot skin-pass mills are many times used for transferring roughness to the strip. In case of hot rolled black strips usually the rolls are not textured due to the presence of scales. The oxides on the surface of the strip are sometimes thicker than the height of crests & valleys of the textured surface. This prohibits transfer of roughness to the strip. Also the roll surface gets loaded with scales / oxides leading to inferior roll surface. The texture transfer (with shot blasted rolls) is mainly carried out on the HRPO (Pickled and Oiled) strips where the product will undergo subsequent coating/painting operation. During skin-passing the work rolls transfers roughness to the strip. Average transferred roughness is around 50% of that of the work roll. The higher is the rolling load, the higher is the roughness transfer.
Q6: Can it be used for improving shape of coil winding, recoiling and dividing the coils?
A6 : During skin-passing the coils are unwound and rewound under tension control of pay-off reel and tension reel. Strip Centre Position Control (CPC) and/or Edge Position Control (EPC) are provided to reduce coil telescopic shape of wound coil. The good coiling accuracy is facilitated by the suitable strip tension, proper work roll gap leveling (tilting) and work roll roughness. The Hot SPM also can be used to rewind and divide the coils.
Q7: Hot skin passing being a finishing operation, what are the major parameters to be considered?
A7 : As the skin-passing is a finishing operation, it is necessary to take in to account certain important parameters to achieve desired results. These are (1) strip temperature and (2) steel aging after skin-passing. Special care needs to be taken concerning the strip temperature which should be lower than 50°C. That is, the coils shall be properly cooled before skin-passing. If the strip temperature of the coil is higher than 50°C, the benefit of the yield point suppression will be lost. The mechanical properties achieved during skin-passing get deteriorated after some days as the free nitrogen and carbon atoms slowly return to the grain boundary producing again a more significant yield point. Hence the skin-passed coils have to go to the subsequent forming process within 10 to 15 days for best results. The process of aging is faster with higher environmental temperature. It has been observed that usually the Aluminum killed steels do not exhibit distinct yield point and hence do not generate Luders lines. These steels are skin-passed for flatness improving.
Q8: What are the different operating modes for hot skin passing?
A8 : The Hot SPM is mainly operated in two types of control modes
– Elongation control mode.
– Roll force control mode
In the Elongation control mode the percentage elongation (reduction) is kept constant by applying constant tension and varying the roll separating force. Under this condition the constant mechanical properties are achieved along the strip rolling direction. This mode is popular where anti-fluttering properties are required. In this mode, shape correction is predominant. The elongation is measured using encoders mounted on the deflector roll and the strip tensions are kept constant. The roll force is varied based on feedback from the elongation measuring tachometers and the elongation set point. Elongation control mode is for constant mechanical properties along the rolling direction.
In the Force control mode the roll separating force is kept constant by opening or closing the gap irrespective of the thickness variation. In this mode, the elongation is achieved by keeping the entry and exit strip tensions constant. This mode is predominantly used when strip shape (waviness) correction is required and texture transfer is important. Roll force control mode is used for strip shape (waviness) correction and texture transfer. Strip elongation is achieved by the roll force and strip tension control.
Q9: Is it possible to add other functions with hot skin passing?
A9 : A Hot SPM can be provided with entry side welder, bridles, stretcher leveller, accumulator, side trimmer, oiler, inspection facility, walking beam conveyors, etc to improve on the yield, quality and production depending upon technical and production requirements.
Q10: What is the presence of Danieli for the hot skin pass mills installations worldwide?
A10 : Danieli is the world leader with maximum number of reference installations of hot skin pass mills. From the initial installations of HSPMs with simple skin passing operation, Danieli have HSPM installations with tension leveling and side trimming functions.
Danieli is supplying the first HSPM for 2100mm wide strips with inline side trimming at Gerdau Acominas, Brazil. This unique mill will process 823,000tpy (1.2mtpy in phase 2) of hot rolled strip of LC and MC, API up to X56, shipbuilding, HSLA, pressure vessel, deep and medium drawing grades ensuring full compliance with final product requirements for suppressed yield points, improved strip flatness, roughness transfer and improved coil shape.
During last three years Danieli Wean United has commissioned two number of most up to date 4-Hi Hot Skin pass Mills with a production capacity of 1,100,000 tons per year each, at Dragon Steel Corporation, Taiwan. These state-of-the-art mills are identical to each other. These mills will produce material to cater the demand of skin passed HRPO coils and HR black coils required for producing pipes, pressure vessels, pressed steel components for automobiles and other applications. Danieli received the order for a third HSPM from the same customer but this HSPM will also include a stretch leveler in line.
The first Danieli HSPM in the country will be installed at JSW Ispat Dolvi plant.
Q11: How does this new technology find importance in India steel industry?
A11 : To meet the continuously increasing demand for higher quality hot rolled coils by the end users in India, the hot skin-passing will become the means of improving shape and properties of hot rolled coils especially with respect to strip flatness, elongation and surface roughness. I have already mentioned that hot rolled coils from conventional hot strip mills or thin slab casting & rolling plants need further processing like recoiling and skinpassing to reduce the rejection rate. When the strip is used to produce pipes, pressed steel components, etc. its use is beneficial to increase quality and reliability of the end product by hot skin passing operation.
Presently the approximate production capacities of HR and CR coils in India are 32 and 16 million tons respectively which leaves big opportunities to install such HSPMs to add value to HR coils with much lower investment compared to CRM complexes.
With a large reference of Hot Skin Pass Mills installations worldwide, Danieli is committed to supply high quality and state-of-the-art plants to meet the present and future requirements of the ever changing market demand at an optimum cost.