Let me start with one question. How does the iron and steel become the primary material in our modern industrial society? Actually, many factors have been making the iron and steel the most competitive material with other structural matters. For instance the abundance of iron ore in the crust of earth and the development of effective process, contribute to the competitive price of steel products. However, even with the price competitiveness, iron and steel have never been used as a basic constructing block of our modern society, if it does not have desired mechanical properties which are desired in many applications. Actually, steel product have a very wide range of properties span, for example, tensile strengths of steel used for beverage can is below 200MPa. But the strength can be increased up to 2,000 MPa, 10 times higher strength, which can be applicable to the cable supporting very huge bridge by addition of small amount of alloying elements. Wide span of mechanical property is unique feature of iron and steel, which is originating from the flexibility of microstructure. Steel may be the only material which has 5 distinctive microstructure constituent which is ferrite, pearlite, bainite, martensite and austenite at room temperatures. Each single phase or the mixture of phases provide wide range of properties which copes with diverse demands from the industry. Therefore, in most of the steel research, microstructure optimization for desired property has been the main subject to obtain a desired product with optimized microstructure. The traditional material development usually follows three primary steps; process, structure and property. For the successful material development, these steps are interconnected to each other as shown in this figure. In the material development, the clockwise direction, process-structure-property chain represent a cause-and-effect logic of scientific flow, whereas the counter clockwise direction property-structure-process link shows a target and means relation of engineering flow. The role of alloying element is basically lying between the process and structure. It can be regarded as an ingredient to make the process window wider which enable us to generate the equivalent microstructure in more convenient or economic way, or to make more effective microstructure for enhanced material property with similar process condition. Let us see one example of applying alloy element for materials design for the development of high-performance steel which is called transform transformation induced plasticity steel, TRIP steel. As you know, in practice tensile test is a common testing method to evaluate the strength and ductility of material. In tensile test, the testing cpoupon is deformed along one direction until the failure occurs. When we measure the stress which can be supported by the material, typically it shows this like curve showing a maximum value at certain point. The decrease of stress is led by the currents of necking which is related to the loss of material ability to keep the uniform deformation. When we deform the material, actually the cross section area of test coupon continuously decreases so the load bearing capacity will decreases which can be considered as a geometric softening. However most of the most of metals have an ability to compensate the geometric softening to carry on the uniform deformation by strengthening itself which is called work hardening. During the tensile test, the necking to failure starts to happen when the rate of geometric softening is larger than that of work hardening. So, in other word, the material can be homogeneously deformed as long as the work hardening rate is larger than that of geometric softening which effectively postpones the occurrence of necking, therefore improves the mechanical properties in terms of uniform elongation as well as the strength. In TRIP steel, the austenite islands in the initial microstructure gradually transformed into hard martensite to obtain persistent work hardening, which is giving better mechanical performance in terms of uniform elongation and the strength. The essence of microstructure concept in the TRIP steel is how we can leave the austenite island in the initial microstructure. To achieve the desired microstructure in TRIP steel, the problem is how we can leave the austenite at room temperature as I mentioned. It is because the austenite is usually stable at high temperature which is likely to transforms into other phases at lower temperature. One of the way to obtain the austenite at room temperature is adding alloying element which can stabilize the austenite at room temperature. The most effective and cheap element for this purpose is carbon. And with this brilliant heat treatment path which is designed to enriche the carbon into the austenite, the carbon content in the austenite can reach to a critical level which is sufficient for stabilizing the austenite. But here we have another problem. As I mentioned, when the carbon concentration increases, it would like to form iron carbide so called cementite and the precipitation of cementite will decrease the solid carbon concentration in austenite which makes the carbon enrichment by the heat treatment useless. That is why the TRIP steel usually contains certain amount of silicon or aluminum which is known to be effective in suppression of cementite precipitation in high carbon containing austenite. With the aid of proper alloying elements, we can obtain this kind of beautiful microstructure containing many austenite island which appears as white features in the micrograph. They eventually contribute to the improvement of mechanical performance of the steel by deformation induced martensitic transformation. In the next section will consider more alloying elements of steel in practice and their generalroles.
