Low Temperature Ductility And Ductile Crack Arrest Properties Of High Strength Low Alloy Steel

By Pankaj Mittal, Welspun Pipes Inc., Little Rock, AR, and Ramesh Singh, Gulf Interstate Engineering, Houston, TX | December 2010 Vol. 237 No. 12

Steel Making
Often it is noted that, at the design stage, the focus is on “pipe” but rarely on the steel. It may be pointed out that a pipe is merely a dimensional aspect of the material it is made from. It is the property of the steel that must be in focus not just the “pipe.” Thus, the steel-making process assumes the center stage in getting the desired finished product (the pipe). The modern steel-making process has changed significantly from the past. Steel makers are more focused on getting ultra-clean steel with finely controlled chemistry.

The current trend in steel making is to keep very low levels of sulfur (S), phosphorous (P) and nitrogen (N). The presence of these elements above certain limits is detrimental to the steel quality, especially to the ductility and crack resistance properties of the steel. To produce clean and fully killed steel, the refining process involves treating the steel with aluminum (Al) or silicon (Si) or a combination of both. Calcium (Ca) is often added to control the shape of inclusions.

Vacuum degassing, by Ruhrstaal-Heraeus (RH), plays a vital role in production of clean steel. The RH degassing process uses an up-leg snorkel through which steel is drawn into the degassing-chamber and the denser degassed steel leaves through the down-leg. It has become a necessary step in steel making, especially for steels which are used for critical applications like sour service and low-temperature applications. The RH degassing system is associated with high tonnage basic oxygen furnaces (BOFs), producing low-carbon, aluminum-killed steel.

RH degassing is generally preferred owing to the metallurgical advantage associated with the downstream refining processes and production of large tonnage of high quality, lower cost, continuously cast steel.

These steps produce clean steel. However, to derive better mechanical properties, the steel is further treated with a very precise alloying process. This is referred to as the micro-alloying process. Some of the key elements that can be micro-alloyed are niobium (Nb), vanadium (V), titanium (Ti), nickel (Ni), chromium (Cr), molybdenum (Mo), and copper (Cu). These elements are used in a predetermined combination to provide the desired mechanical properties. Steel producers have different combinations as per their specific fine-tuned procedures and techniques to get the desired end results.

One of the key aspects of the term “clean steel” is the importance of segregation control. The modern automatic continuous casters are now equipped with a dynamic soft reduction technique which helps in minimizing the center line segregation. The steel suppliers are achieving segregation control to level 2 or better on the steel mill’s scale of 1-5 checked by macro etching the cross section of the slabs. Steel mills have developed their own standards and leading manufacturers have developed very good standards. Several other steel mills use the same or similar references of their own to classify the level of segregation in their products. Figure 3 shows the centerline segregation in a steel slab.

Figure 3: Centerline segregation in steel slab