The Science of Forming


 

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Moving Steel Developments from the Lab to the Pressroom

By: Stuart Keeler

Wednesday, February 1, 2017
 

The manufacturing world demands stronger steels with improved formability, energy absorption, corrosion resistance, defect reduction and other properties. Searching the historical technical literature, one finds that these same struggles began in the 1940s, as evidenced by Dr. Jevon’s book, “The Metallurgy of Deep Drawing and Pressing.” Concurrently, Professor H. W. Swift, of Sheffield University in England, gave the 23rd Lecture at the Institute of Metals in England, titled “On The Foot-Hills of the Plastic Range.”

The two mountain peaks represent the respective knowledge bases of steelmakers and users. One learns about the topics by climbing the mountains; the space between represents a lack of communication.
Swift established a series of theoretical hills (see accompanying illustration) that metallurgists needed to climb as they studied the huge volume of historical knowledge published by prior metallurgists. Not restricted, the metallurgists now understood the basis of existing science. Starting from the foothills, each higher area of the hill represents increased success. Their advanced information (1900s) still finds use today:

  • 1980s—surface topography
  • 1970s—strain-rate hardening (m-value)
  • 1960s—forming-limit diagram (FLD)
  • 1950s—plastic anisotropy(r-value)
  • 1940s—workhardening exponent (n-value)

During the same time periods, press designers, die makers, lubricant suppliers, steel buyers and others (the peak on the right-hand side of the illustration) were not searching historical past work for new scientific capabilities. Instead, they used whatever science was on hand for ease of press-shop forming and trial-and-error experimentation.

The 1990s ushered in a new era of useful tools. Universities and other technical institutes were using a computer program that mathematically predicted the behavior of crashing items dropped from planes. Different names were used for more descriptive tools: virtual analysis, computerized forming simulation, computerized die tryout, etc. The key to success was inserting the correct material properties into the program and using the FLD as the forming limit.

During the same time period, a 5-yr. research project allowed a team of engineers and scientists from automotive and steel companies, universities, and other sources to determine the exact change of coefficient of friction of a material being formed in a die. Knowing these numbers would make formability analyses more accurate. While some test results proved useful, the primary goal of the project failed—the coefficient of friction changed so quickly for every location in the die-to-workpiece interface that the researchers could not measure the active numbers.

Later during the 1990s, new advanced high-strength steels (AHSS) began to appear in different parts of the world. AHSS products differ from high-strength low-alloy (HSLA) steels in that rather than containing a single phase (ferrite) in their microstructure, AHSS grades contain a host of phases: martensite, bainite, retained austenite, etc.

An advantage of AHSS grades is their ability to increase the workhardening exponent (n-value) early during deformation by inserting approximately 10-percent martensite particles. At the start of deformation, the martensite interacts with the ferrite, causing the generation of a new n-value. The higher n-value prevents dual-phase (DP) steel (an AHSS grade) from forming localized strain gradients and causing early failures.

Another AHSS grade, transformation-induced-plasticity steel, contains martensite, bainite and retained austenite, providing a very high n-value and high yield and tensile strengths. These alloys prove useful for stamping high-deformation products.

The Future Has Arrived

Note the large gap between the two peaks in the illustration, which represents a lack of communication between steelmakers and users. Efforts to close this gap are being made by newly minted “formability engineers”—people who have completed university-level instruction and on-the-job training, and have experience in metalforming.

And, steelmaking technology continues to evolve. Examples: While continuous-annealing lines have traditionally used water during processing, different liquids are being evaluated; and, researchers have found that inserting chunks of hard martensite into an HSLA steel can create a DP steel. However, when these chunks are on a trim edge that will be stretched, notched, bent or otherwise subjected to a tensile stress, there is a high chance of cracking and tearing. A solution for this cracking problem is being attacked right now, at M.I.T.’s Center for Nanoscale Projects. MF

 

Related Enterprise Zones: Materials/Coatings


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