Cold Sheet Rolling, Processing, Coating & Finishing
Third-generation advanced high-strength steels (AHSS) contain significantly high quantities of alloying elements. Many of these alloying elements create oxides in the surface scale of hot band that exhibit poor solubility in traditional pickling baths. The work presented here examines hot band samples of various quench and partition grades of steels and the effect of adding a fused salt conditioning step to the descaling process to both reduce the overall pickling time required and to improve the final surface quality of the metal. Also presented here are conceptual design elements for a production line possessing fused salt conditioning.
A series of tests has been conducted to study the nature of residual stress patterns and relaxed flatness in parts cut from temper/roller-leveled cut-to-length heavy-gauge plates. Testing was performed on both 2-high and 4-high temper mills, with both tight-line and tension decoupled interconnects to the levelers. The results indicate a through-thickness stress yielding/reprogramming by the temper mill may not be in effect, and that a high-strength, compacted crust constrains the retained, non-yielded interior stresses. This paper examines the approach taken, the results and how these implications direct the operational concepts of the line.
New advanced high-strength steels (AHSS) require higher annealing temperatures, low-dewpoint atmosphere, faster cooling rates, better strip shape and uniform temperature. With the development of harden and temper lines combined with the design features of several other types of processing lines, the ability to process various grades of AHSS has been achieved. These full-size coils can now be utilized for real-world product development needed for the automotive industry.
Cold rolling lubrication trials were carried out on a high-speed reversing pilot mill. With low-alloy hot-rolled steel, significant differences were found in friction and strip cleanliness between three different emulsions, comparable to the performance on a production mill. Moreover, the trials show that the coefficient of friction clearly decreases as mill speeds increase. The corresponding variation in lubrication regime correlates well with strip surface texture on the microscopic scale. The mill trials were further carried out with a hard, alloyed steel type. Better strip cleanliness was found, but the differences between the two emulsions remained, both in terms of cleanliness and friction.
One of the goals in the pickling process is for the steel strip to remain immersed. It is crucial to control the depth that the steel strip assumes in the bath inside the tanks. In order to achieve this, a reliable measurement of the catenary formed by the steel strip must be done. Several issues in this process make optical measurements and physical contact methods almost impossible or at very high maintenance costs. This paper presents a solution that performs this function without contact with the acid solution and discusses the advantages of a low-cost solution to quality and production issues related to the steel strip movement and catenary during the pickling process.
The forming and joining behaviors of galvanized sheet steels are integral to their usefulness during processing and end-use performance. Historically, the ease and speed of forming and joining operations of these steels have been a major cost advantage over competitive materials and in many cases have enhanced performance, for example the use of work hardening and texture control during forming. Continuously increasing requirements for higher levels of mechanical properties with thinner sections has led to the adoption of hot-press forming, for which the use of a zinc-based coating is highly desirable to provide needed galvanic protection. A better understanding of the nature of galvanneal (GA) coatings and post-treatments has helped to improve forming performance of GA-coated sheet steels. Zinc-magnesium-aluminum hot-dipped coatings show forming advantages over galvanized. An understanding of factors influencing edge fracture in multi-phase steels has led to improvements in forming behavior of these steels.