What is the law of performance change of hot-rolled bars during the subsequent heat treatment process?
Publish Time: 2025-03-19
As an important product form of special steel, hot-rolled bars are widely used in machinery manufacturing, automobile industry, aerospace and other fields. Although the hot rolling process can give the bar certain mechanical properties and microstructure, in order to meet higher use requirements, the subsequent heat treatment process is often indispensable. Heat treatment can significantly improve the mechanical properties, wear resistance, corrosion resistance and microstructure uniformity of hot-rolled bars by controlling the heating, insulation and cooling process of the material.First of all, the heating stage in the heat treatment process has an important influence on the performance of hot-rolled bars. Hot-rolled bars usually have certain residual stress and uneven microstructure after rolling, which will limit the further improvement of their mechanical properties. Through heating, the atomic activity inside the bar is enhanced, the grains are rearranged, and the residual stress is released. The choice of heating temperature depends on the chemical composition of the material and the expected performance target. For example, for low carbon steel, the heating temperature is usually controlled above the Ac3 line to ensure complete austenitization; while for high alloy steel, the heating temperature may need to be higher to dissolve alloy carbides and achieve homogenization.Secondly, the holding stage is a key link in the transformation of microstructure during heat treatment. During the holding process, the grain size and distribution of hot-rolled bars will change. For materials that require grain refinement, appropriate holding time can promote recrystallization and form a fine and uniform grain structure, thereby improving the strength and toughness of the material. For example, in normalizing treatment, too long holding time may lead to grain coarsening, which in turn reduces the mechanical properties of the material; while too short holding time may lead to incomplete organizational transformation and affect the uniformity of performance. Therefore, the selection of holding time needs to be precisely controlled according to the characteristics of the material and the heat treatment objectives.The cooling stage is the link in the heat treatment process where the performance changes most significantly. The cooling rate directly affects the microstructure and mechanical properties of hot-rolled bars. For example, in quenching treatment, rapid cooling can transform austenite into martensite, thereby significantly improving the hardness and strength of the material. However, too fast cooling rate may also lead to increased internal stress and even cracks. Therefore, the selection of quenching medium (such as water, oil or air) and the control of cooling rate are crucial. For some special steels, such as tool steel or bearing steel, tempering treatment after quenching is essential. Tempering can eliminate the internal stress generated during quenching through proper heating and heat preservation, and adjust the hardness and toughness of martensite, so that the material has good plasticity and toughness while maintaining high strength.In addition, the phase transformation behavior during heat treatment also has a profound impact on the performance of hot-rolled bars. For example, in the quenching and tempering treatment (quenching plus high temperature tempering), the microstructure of hot-rolled bars changes from martensite to tempered troostite, which not only has high strength, but also has good toughness and fatigue properties. For some high-alloy steels, secondary hardening may also occur during heat treatment, that is, the hardness and wear resistance of the material are further improved by precipitating fine alloy carbides.It is worth noting that the performance changes during heat treatment do not occur in isolation, but are closely related to the initial state of the hot-rolled bars. For example, the banded structure or segregation phenomenon formed during hot rolling may be further aggravated or alleviated during the heat treatment process. Therefore, when formulating the heat treatment process, the initial organizational state of hot-rolled bars needs to be fully considered to avoid the genetic effect of undesirable organizations.In general, the law of performance change of hot-rolled bars during subsequent heat treatment is a complex and systematic process involving the interaction of multiple stages such as heating, insulation and cooling. Through reasonable heat treatment process, the mechanical properties, microstructural uniformity and performance of hot-rolled bars can be significantly improved. However, the optimization of heat treatment process needs to comprehensively consider the chemical composition, initial state and use requirements of the material to maximize the performance. In the future, with the continuous advancement of materials science and heat treatment technology, the performance regulation of hot-rolled bars will be more precise and efficient, providing stronger support for the development of high-end manufacturing industry.
