Titanium Microalloyed Steel: Fundamentals, Technology, and Products

In 1963, the Swedish Noren first proposed the definition of microalloyed steel, namely the Mn-bearing alloy steel or low-alloyed steel with the addition of a small amount of alloying elements. The alloying element has a significant effect on one or several properties of steel, and its amount is smaller than that of traditional alloying element in steel by 1–2 orders of magnitude (Noren in Special report on Columbium as a microalloying element in steel and its effect on welding technology. Ship Structure Committee, Washington, 1963, [1]). This definition has been widely adopted around the world and has been in use up to now. Titanium microalloyed steel is such one kind of microalloyed steel, and titanium is a typical microalloying element. There are other similar elements, such as niobium, vanadium and boron.

The main roles of the titanium in the titanium microalloyed steel are the grain refinement strengthening and the precipitation strengthening. The smelting of titanium microalloyed steel should satisfy that the most of the titanium dissolves in the molten steel, and precipitates in the form of carbide or carbon nitride after the subsequent solidification, rolling and heat treatment processes. Due to the fact that the affinity between titanium and oxygen is less than that of aluminum and oxygen, but stronger than that of silicon or manganese and oxygen, the addition of titanium can lead to the formation of large amounts of Ti-containing oxides in case of the molten steel is not properly deoxidized during the smelting process.

The precipitation of microalloying elements is one of the most important issues in microalloyed steels. It is well recognized that controlling the precipitation process of microallying elements in steels is an effective means to significantly improve the strength of steel material due to precipitation strengthening and grain refinement by controlling the austenite grains coarsening during reheating process and recrystallization process. Moreover, controlling the precipitation behavior of secondary phases in steel, leading to an accurate control of volume fraction, shape, size and distribution of precipitates, could effectively improve the microstructure and mechanical properties, which is a significant issue for microalloyed steel in the field of theory research and production practice.

High-strength Ti microalloyed steels are mainly precipitation hardened ferritic steels. The good combination of ferrite grain refinement and TiC precipitation strengthening plays a key role in obtaining both high strength and high toughness simultaneously for those steels. The ferrite grain refinement, on the one hand, depends on the refinement of austenite grain size and on the other hand depends on the control of transformation temperature. The refinement of austenite grain size mainly depends on the control of the austenite grain growth before hot rolling, and the morphology and size of the recrystallized austenite grains during hot rolling.

As a kind of microalloying element, titanium significantly improves the comprehensive properties of steel. However, when compared with niobium and vanadium microalloying technology, Ti-microalloying technology had not been extensively used in industry for a long time. It is because the properties of Ti-microalloyed steel fluctuate largely and the production process is not stable. Titanium is very active and tends to react with oxygen, nitrogen and sulfur to form large Ti-bearing phases which are harmful to the comprehensive properties of steel, such as TiO, TiS and Ti₂CS. More importantly, the formation of these phases consumes a portion of titanium. This consumption not only reduces the volume fraction of TiC precipitated at low temperatures, but also significantly changes the chemical free energy of TiC precipitation. Therefore, the precipitation behavior of TiC is remarkably changed and its strengthening effect is greatly affected. In addition, the precipitation of TiC is sensitive to temperatures and the variation of production parameters remarkably affects the properties of steel. As a result, the mechanical properties of Ti-microalloyed steel products of different batches with the same chemistry or even different locations of the same batch fluctuate greatly.

In the 1920s, titanium was used as a microalloying element. Initially, it was used in the trace-titanium microalloying treatment to improve the welding performance of steel. With the deepening of the research on the effect of titanium in steel and continuous progress of technology, such as smelting and rolling, the role of titanium steel is further highlighted and the product range is been continuously broadened. Representative products mainly include German QStE series of steel (titanium content ≤ 0.16%), YS-T50 produced by Youngstown Sheet and Tube Company, automobile beam steel NSH52T (titanium content: 0.08–0.09%) produced by Nippon Steel and so on. The research on the titanium microalloying technology and product development in China launched late and the first titanium microalloyed steel was not developed until the 1960s. The representative product was 15MnTi (yield strength: 390 MPa). Then, a series of products were gradually developed, such as the hull structure steel 14MnVTiRE (titanium content: 0.07–0.16%), automobile beam steel 06TiL, 08TiL and 10TiL (titanium content: 0.07–0.20%) and weathering steel 09CuPTi. After 2000, many researches were carried out on thin slab casting and direct rolling (TSCR) process, titanium microalloying technology and high-strength steel, and a great leap forward development was achieved in China. A series of 450–700 MPa grade titanium microalloyed high strength steel products was developed, which is mainly applied for the container, automobile, construction machinery and other industry fields. This chapter mainly introduces the representative achievements of titanium microalloyed high strength steel in China.