Resumo:
Stretcher reducing mill (srm) is a mill that defines the external diameter of tubes. it is composed by 24 stands and each stand uses 3 rolls. the final external surface quality is achieved in the srm and any undesirable external marks in this process is hardly recovered. a great analysis was developed to understand the mechanism of external marks. the first step was to check the rolls quality based on technical requirements of rolls. a characterization was performed to check not only the requirements in the specification but also the material integrity. an opportunity was identified about the specification of roll. based on results of characterization, next step was to communicate the main supplier about the result of characterization. The hardness was out of specification (lower) and an action was required. since the supplier did not accept the claim in a first view. The third step was to confirm in vallourec group all specification of rolls concerning hardness of rolls. It is an important and critical requirement of roll strongly linked with wear resistance. The majority of rolls in Vallourec group has the same range of hardness. therefore, the specified hardness was preserved and MORE COMPLETE SPECIFICATION was made taking ALL THE KNOWLEDGE GAINED IN the characterization phase. The fourth and hardest step was to develop a new supplier in an internal market with a low know-how in manufacturing rolls. the casting process was developed after some tentative and thermal treatment was also required to achieve the specified hardness. an unusual heat treatment was specified in partnership with an expert supplier chosen by vallourec once the partner of foundry was not able to heat treat the material without breaking the material. The vallourec partner developed a curve and process able to heat the material and achieve the hardness with very low variation. the last step was to apply a pilot batch to qualify the suppliers (foundry and heat treatment). The results were acceptable and the external marks were minimized when using the new rolls. However, srm uses lots of stands and many rolls with low hardness are still working. all suppliers were pushed to improve their quality issues. so, a new level of quality was established with main requirements for rolls. No thermal treatment and chemical analysis was specified in order to not limit the suppliers technology of materials.
